With the release of the long-awaited DLC (Coloring Game 2 - Deluxe Edition), you can now add your own images to the game.

https://store.steampowered.com/app/1221790/Coloring_Game_2__Deluxe_Edition/

UPD: I recommend using pixel images without increasing or losing quality (JPEG).
A good resource is - http://pixeljoint.com/

If you need to reduce the image, crop it, then you have to use third-party programs.
For example, an online editor - https://fotoflexer.com/editor/