First you go to Paint. Please save as 24 bit-bitmap, so you don't lose the quality of the image! Then, after you convert to gif format (not with paint) cause there will be bad quality, and that would not look good in the game. That looks like 16 bit quality, and that's pretty shity! In editart you press: Backspace and then the key C, that makes pink color background. That will be in game transparently. Not Pink color. You can copy the color when you press key TAB. The same as you copy sprites&textures in Build mapedit program, by Build Engine! You press again Key C to apply the copied Color. You also can make, to change the Brightness/Contrast in editart, to sprites/textures.
Regards. Good luck!