Convert Image To Doom Palete

I was using GIMP to make a PNG image have a transparent background (it was going to be a pickup in Zdoom) and the edge of the picture would be discolored in game. Is it because it is is not in the doom palete or for some other reason. How do I go about converting it to the doom palete.

If this helps this is what it should look like:
http://rapidshare.com/files/318699536/bandoleer.png

The original picture looked like this:
http://www.quake2.com/q2wfaq/bandoleer.gif

try maybe change the background to purple RGB 255, 0, 255
are you using xwe to import it?

I am using SlumpEd for images and XWE for everything else.