Pixabay – Free to use any way you like, with no acknowledgement required. Photos are contributed by users and as such, you are relying on their confirmation that the images are theirs to share (although the site moderator reviews all contributions). Pexel seems to operate similarly and also turns up a lot of images from pixabay.
Emojipedia – emojis evolved from the text-based pictures created by users such as a colon : plus a right-hand parenthesis ) to represent a smiley face :). They are now standard picture-based symbols that are built into standard operating systems and work across all platforms to render similar (but different) pictures. This site provides a repository of the emojis and shows you how they will appear in different operating systems. They are free to use and do not require attribution.
Find free to use images – search Google and filter the results, using Free to use share or modify: Allows you to copy, modify, or redistribute in ways specified in the license.
Freepik – Free to use commercially with acknowledgement.
Open Clipart – the site owners claim they use the Creative Commons Zero 1.0 Public Domain License (CC0) every time an artist uploads a piece of clipart to Openclipart to make it clear the artist is releasing the creative work for anyone to use for any reason, even commercially.
Designers pics – big close ups and arty landscapes at high resolution for personal & commercial use and apparently no attribution required.​
Jay Mantri – stunning pics made available under CC0.
Unsplash – free to use and categorised.
Gratisography – photographed by Ryan McGuire and free of copyright restrictions.
Free images – free as long as you stick to the rules in the Image license Agreement. Also, in some cases you may need to notify the artists about using the images and sometimes you need to give credit to them.
FirmBee – Free PSD mockups – drag and drop your design inside the smart object, make adjustments if needed and you’re done.