Spriting with GIMP
This is GIMP.
GIMP is completely free and one of the few programs that can be used to create sprites for CCCP, but it's often a daunting program to learn for newcomers.
The upside is that you need less than 1% of what GIMP's capable of in order to sprite like a pro for CCCP, and this tutorial has an image for every step, but if you get stuck please do tell me where on Discord (#MyNameIsTrez1585) so I can prevent future people from getting stuck. :)
The reason why you should use GIMP instead of Paint/Paint3D/Paint.net/etc. is because those other programs don't support indexed color mode.
If your image isn't in indexed color mode, it'll look corrupted in-game.
Pixels in most images are stored using one byte (256 values) for the red channel, one for the green channel, and one for the blue channel, totaling 3 bytes (16 million values).
CCCP's indexed images however only use 1 byte per pixel. This means its images are thrice as small, but it also means that you can only draw sprites with the 256 colors of the game's palette.
Let's say we want to draw this sprite:
Right-click this empty spritesheet and select Save image as...
Note: DO NOT use the Copy image option since it doesn't retain the image's palette, breaking the image.
Drop the downloaded image on top of GIMP to open it.
Zoom into the color palette area in the top-left of the image by holding the Ctrl key down and scrolling up and down with your mousewheel.
Pick a color to draw with by clicking the Windows button at the top of GIMP, then Dockable Dialogs and then picking Tool Options at the top.
