These are Creative Commons Zero (CC0) images, which means you can copy, modify, distribute and use the photos for free, including commercial purposes, without asking permission from or providing attribution to the photographer.
My “go to” sites for photographs are pixabay and unsplash. Another site might have a different or more specific image that you are looking for from the list below. If you know of a site not listed here please let me know.
Archive.org Source of free videos and images collected by the non-profit waybackmachine. Be aware that individual files on this site have different use permissions and restrictions, determined by the creator; please respect the ownership and rights fot he media you want to use in your work.
Barn Images - Two photographers from Latvia, Igor Trepeshchenok and Roman Drits offer you a collection of free high-resolution non-stock photography.
BucketListly Photos - A free creative common collection of travel photos anyone can use. Brought to you by Pete R., founder of BucketListly.
Clker is the biggest library for pure and reliable Public Domain clipart, mostly SVG graphics. There's a ton of drawings on virtually any topic.
Creative commons - search across various sites for public domain content at once including flickr
Creative Market - Has a "happy monday" thing where every monday they give you 6 free photo/typefaces/artwork.
Cupcake - Photographer Jonas Nilsson Lee offers free images to the public domain. That means no attribution ever unless, of course, you want to.
Death to the Stock Photo - Free monthly photo packs for creatives. A photo & inspiration haven for creatives crushing their path.
The world's largest creative designer community. The exotic and artistic images and collages are published under various user selected licenses. It's a stunning resource, but be careful and read the requirements for each image individually.
Epicantus - Hi-resolution photos added every week by visual and UX designer and lean startup coach Daria N.
You can't just use any image on flickr but use extended search and select the option "Creative Commons" to get appropriate images, which all require proper accreditation.
FoodiesFeed - A delicious resource of free realistic food images in high resolution and free digital goods related to food.
Free Nature Stock - A project by the talented Adrian Pelletier, providing completely free nature photos of some absolutely beautiful locations, and the best part is that every single day there's a brand new photo added.
FreeImages - Are free as long as you stick to the rules in the Image license Agreement. In some cases you may need to notify the artists about using the images, See restrictions under the image previews, right next to the Download button.
Gratisography - High-resolution photos by Ryan McGuire you can use for free on your personal and commercial projects.
Collection of free SVG icons and the ability to upload your own vector graphics to be converted to web icon fonts.
A collection of creative commons SVG graphics available for download.
IM Free - A curated collection of free web design resources including photos, all free for commercial use.
ISO Republic - Tom Eversley, a designer and photographer from England provides exclusive free and premium stock photos for creatives.
Jay Mantri - Jay Mantri is a graphic designer and photographer that captures amazing photos specifically for use within design projects.
Kaboompics - A great place to get breathtaking free pictures for business or personal projects.
Life Of Pix - Free high-resolution photos under public domain, mentioned CC0 licence. Photos are provided by Leeroy advertising agency in Montreal and its network of photographers.
MMT - Jeffrey Betts provides free photos for personal and commercial use under CC0 license.
About 200,000 photos free for almost arbitrary use under proprietary license. Finding the best shot can be a bit tiresome and navigation needs getting used to.
Moveast - A collection of a Portuguese guy traveling the world and generously sharing his photos for free.
NASA Creative Commons NASA Images are generally nor copyighted although they will occasionally link to images that are and use them with permission. Those images are marked with a copyright symbol and authors name. Do not use these image unless you have permission to do so.
Negative Space - Free stock photos added every week. All photos are released under CC0 licence.
Peekspace - Patrick Goethe collects and curates free space photos scattered around the web.
Pexels Free stock photos you can use everywhere from various resources under the CCo license. Free for commercial use. No attribution required.
Photo Collections - Another free stock photo project by Vadim Goncharov. He openly invites you to use his photos on personal or commercial projects all for free.
PhotoStockEditor - A growing collection of free high-resolution images for personal and commercial use.
Picjumbo - A free stock photo site created by designer & photographer Viktor Hanacek in 2013. It all started when any stock photo site didn‘t want his photos due to lack of quality.
Picography - Free hi-resolution photos with the "use them however you like" licence provided by Hidden Depth.
Pixabay a repository for stunning public domain pictures. Your source for free vectors, free drawings and free photos. You can use any Pixabay image without attribution in digital and printed form, even for commercial applications.
Skitterphoto - Amazing collection of high-quality and vivid photos taken by three skitter photographers residing in Groningen, The Netherlands.
Snapwire Snaps - Get 7 free beautiful photos every 7 days by Snapwire. Commissioned photos from 200,000 of the world's most talented photographers.
SpaceX Photos - Exclusive collection of promotional SpaceX stock photography available for free on Flickr.
Startup Stock Photos - Simple, trendy photos of the startup environment and equipment. An excellent resource for startups, bloggers, publishers, websites, designers, developers, creators & everyone else.
Stock Up - Dynamic free stock photo search engine with 11,233 indexed at the time of writing.
Stocksnap - Hundreds of high-resolution images added weekly. All photos are free from copyright restrictions, meaning no attribution is required.
The Pic Pac - Authentic photo packs with "do whatever you want" licence. All packs are "pay what you want" meaning you can get them for free or support the project by donating any amount of money.
Tookapic - Essentially, a platform for starting your "photo a day" project, Tookapic has a marketplace for stock photos and a huge selection of free images.
Travel Coffee Book - A large collection of beautiful travel moments available as CC0 photos.
Unsplash - One of the greatest resources for free photos. Over 50,000 of real and honest photos contributed by talented people.
About 20,000,000 images and diagrams under various licenses; also public domain, but mostly attribution is required. The quality of available images is very inconsistent and it can be hard to find useful works.