When discussing sprite editing for dungeon Keeper on Discord, TheSim came up with this Dungeon Keeper palette:
I'm not sure how he got is, but it looks right at first glance.
Unfortunately, there are only 247 colors on it, so I don't know how accurate it is.
If some one can find a 256 color palette, please post it in this thread.
So in checking in the Gimp on how to use it, I made this custom Gimp palette, that you can install in Gimp by putting it in:
C:\Program Files\GIMP 2\share\gimp\2.0\palettes
That's the palettes directory of Gimp, where all the default palettes are stored. If you put it there, it will be one of the palettes to choose from for indexed colors.
Dungeon_Keeper_palette.zip
Alternatively, you can make the palette yourself from the image above.
A sprite made in RGB. should than be converted to indexed colors using that custom palette.
Converting between indexed and RGB in Gimp is done by Image > Mode > choose new mode.
Very important menu for image editing, so check image > mode, if you want to improve your power over image editing.
Indexed formats are gif, certain BMP's, not sure which more. Mostly, image formats are in RGB.
In general, drawings compress better in gif, photo's compress better in jpg, but these days mostly png is used.
At any case, don't convert sprites to jpg, that will do a lot of damage. Gif is the most accurate compression.