Free Stock Photos, Icons, and More!
I have compiled this page to act as a resource for finding free stock photos. Remember to keep some things in mind whenever you are using stock material. Namely, you should always credit the original creator if possible, and you may be required to do so in certain situations. If you're just using photos for personal reasons, I doubt you'll get in any trouble, but if you're using them in your content on websites, in videos, or other commercial media make sure you have the proper license to use them.
When in doubt, do a reverse image search on the photo. This will help you determine if it's copyrighted.

Stock Photos
You've probably heard of ShutterStock. We will not be talking about them here. Instead, here are some of my favorite free stock photo sources you can use. Many of them are used on this website!
Unsplash
Attribution Appreciated - Not Required
Unsplash offers an enormous catalog of free, royalty-free stock photos. If you need a picture of just about anything you can think of, you'll probably find it here. The vast majority of photos are of high quality and high resolution. Attribution is appreciated but not required, and you can use the photos in commercial works. You are free to modify or use any of the photos as you see fit. The only thing you're not allowed to do is sell the photos themselves or use them to start your own stock photo website.
They do have paid photos available with Unsplash+, their premium tier, mixed in with the free ones. So watch out for that.
Anyone (with decent photos) may submit their creations to Unsplash for the public to use. I love it!
Pexels
Attribution Appreciated - Not Required Video Too!
Pexels, which looks like the word pixels with a typo, is another great resource for completely free, royalty-free stock photos. Their license is pretty much the same as Unsplash. In fact, they may pull some of their photos from Unsplash.
You're allowed to use the photos for whatever you want, without attribution, so long as you're not just reselling the photos or placing them on other stock photo websites.
One nice thing about Pexels is they offer free stock video in addition to images.
Wikimedia Commons
Attribution May Be Required
WikiMedia Commons is like Wikipedia's stock photo archive. It's where you can find all the diagrams, photos, etc. across Wikipedia, uploaded by random members of the public for free. One big thing to note with WikiMedia Commons is that many of the images require attribution. So always check the license for the image you're interested in. A lot of images are public domain as well, so no attribution is required on those. Many images are licensed under the Creative Commons license.
DeviantArt
Attribution Often Required Some Require Purchase
DeviantArt is a massive repository of content created by artists all over the world. Many (but not all) of the assets are available for free. Attribution is often required, and you should always check with the artist directly before using things commercially. You may have to pay for commercial use. It's also a great place for finding inspiration.
Microsoft Office
Limited To Office Applications
Microsoft Office actually has a lot of nice stock photos you can access. There is one extreme limitation, however, you are only allowed to use stock photos from Office within Office applications. That means it's okay to use them in your PowerPoint presentations and Word documents, but you aren't allowed to copy-paste them into other programs, or use them on your websites, for example.

Stock Icons
There are many stock icons available. These are meant for interface designers and websites, but you can use them in other places too. Many don't require attribution, which is nice!
Font Awesome
Attribution Appreciated - Not Required
Font Awesome is an impressive set of icons you can use on your websites (or elsewhere). The free version includes over a thousand icons, but they also offer a paid version with several thousand more. The free icons should suffice for most uses. Attribution is not required (but appreciated). This website uses many Font Awesome icons. The sample icons above this section are from Font Awesome.
Boxicons
Attribution Appreciated - Not Required
Boxicons is very similar to Font Awesome and 100% of their icons are free. Attribution is appreciated, but not required.
Ionicons
Licensed under MIT
Ionicons is a third great choice for icons. They're licensed under MIT, which allows you to do whatever you want with the icons, provided you include a copy of the license with your distributions. Note that I had difficulty working with these icons in Adobe Illustrator, but they seemed to work fine in Inkscape. These look pretty sharp and appear slightly more detailed than the others, which could be a good or a bad thing, depending on your application.
Icons8
Double Attribution Required Some Require Purchase
I was a little hesitant to put this one in here, as they're a bit more strict with their free license than the others. They offer their icons for free, but you have to pay for their premium service if you want to download the vector graphics on most sets. You also have to put two links back to their website on each page you use their icons on. That said, Icons8 does have a ton of icons. Tens of thousands. Far more than the other sites listed.

Illustrations
The stock photo websites listed above largely don't allow illustrations. Things like clip art, vector graphics, diagrams, etc. can be found on the following sites.
Openclipart
Public Domain - No Attribution Required
Openclipart has thousands of free SVG illustrations/clip art licensed under public domain. So you're free to use and modify as you want. I've used many items from this site in illustrations on this website. I've even contributed some of my own, if you'd like to check them out.
unDraw
Open Source - No Attribution Required
unDraw is a great resource for free open source illustrations. They have several hundred clean looking, modern, minimalistic illustrations with characters doing everything you could imagine. You may modify and use them however you would like, so long as you aren't just reselling or redistributing the illustrations by themselves.
The illustration of characters on a couch above is from unDraw.
DrawKit
Free - No Attribution Required Some restrictions on merchandising Some Require Purchase
DrawKit offers a combination of free and paid illustration sets. Dozens of the sets are free to use commercially. They have some icons as well.
Mockups
If you want to create mockup photos, like fake 3D Books with your logo or something, these are some great free mockup websites. Just pick your template, upload your picture, and it will generate a mockup for you.
Note that I couldn't locate any formal licenses or attribution requirements on any of these websites. They do however say 100% free or Always Free, so I assume you may use them commercially without attribution.
DIY Book Covers
100% Free
DIY Book Covers offers a selection of a few dozen different book covers. Just upload a picture you want to use as a cover, and it will generate then allow you to download the mockup image.
Mockup Photos
Always Free
Mockup Photos offers a bit more options than diybookcovers. Instead of just book covers, they have other things like mockup billboards, graph paper, phone screens, etc. All their templates are free as well.
Covervault (Photoshop Required)
100% Free
Covervault has a ton of great-looking 3D mockups that you can use if you have Photoshop. A nice gentleman named Mark made them and released them all for free. They're Photoshop templates. You download them, save your cover file in the correct location, and Photoshop generates the mockup for you. Mark's a good guy. Thank you, Mark. The fake book and fake phone above are from this site.