Sprites are just pictures usually 32 by 32 pixels made with gimp or paint shop pro. A sprite can also have a power of 2 meaning that it can be 2 by 2, 4 by 4, 16 by 16 ext. It not really important with Dark GDK to have them exactly this way. It just saves memory if you compact them in this format. I would not worry about programming yet get your graphics done first, and maybe program later.
You can always sell your art if your good to TGC, but you first have to build a good portfolio. It is harder to sell code then art for game design. So start out using Gimp it is free and easy to use, and take some art classes to see if you like art. Actually you really don't need to be that good at art if you make a sprite sheet you probably can sell it cheep.
Developer of Space Chips, pianobasic, zipzapzoom, and vet pinball apps. Developed the tiled map engine seen on the showcase. Veteran for the military.