Shimor

You know, if you still have the original image (or another version) in full quality, you can probably convert it again to 256C with less quality loss.

If you have the image then you should look for a function in the paint application you use that looks something like this: Decrase colour depth > 256 colours > and then play with the options since they have varied results depending on the picture (try "optimized median cut > nearest colour" or "optimized octree > nearest colour").