mtpaint should go in a "Okay" list, I think. It gets the job done, but it lacks tools and has a few problems. It's very easy to use and does color palettes very well, however it does not have any options for shifting your colors along the palette even though it has several for shifting and rearranging your palette. It has some options for animations but they don't seem to work in a way that is compatible with any games. You can make sprite-sheets in mtpaint and those work well. If you select something, it selects the transparency as well, and the transparency will overwrite things underneath. I guess mtpaint works with only one layer and it doesn't give an option to let things show through the transparency. This creates additional problems when you try to carefully use the polygon selection tool to trace around the image, because the polygon selection tool grabs extra pixels along the right side of the piece. But it doesn't do it in a very consistent way. It is extremely difficult to figure out how to draw your selection such that you get exactly the pixels you want, all of them, and no others.
Overall, mtpaint is tedious for anything but the simplest work.
Have any of you used Evilpixie or Grafx2? Could you tell me how they measure up as far as working with images and sprites in openxcom? I want something that is easy to use but also offers several tools in case I want to get down and dirty and make complex graphics.