This site contains lots of flashy stuff and eyestrain in some parts. Beware of animation and flashing colours! Oh, and JUUUST in case... Do not take anything on this website seriously. ESPECIALLY the "terms of service".
I use and edit lots of free assets, make my own from scratch. Here are some useful places that I visit the most. I will add more overtime :)
W3Schools Photopea PixelSea TextureTown The 88x31 GIF collection Free Backgrounds Gifcities RV's free Javascript/DHTML effects Ditherit