Here are two sample images, to test painting programs :
"15" is written in color 15, "255" is written in color 255, and the background is color 0.
It's the battlescape palette, so those three colors are black.
The first image has color#0 tagged transparent so you'll see the digits on your browser, but they are otherwise identical (OpenXcom doesn't care if your color zero is tagged transparent or not) If your painting program can open these image, save them, reload them and STILL has 3 distinct black colors in the right indexes, it should be ok for OpenXcom modding.
Edit: You can also add
Evilpixie to the "Good" list. If you want something simple and straight to the point, it's it. Paint with two mouse buttons, mouse wheel to zoom (and third button to drag view), keyboard shortcuts for every tool.