must be the colours you're saving it in.
If you get the original login image out of the file, keep it in its current colour/image format...load it in paint, then import your login image in paint (so it's on top of the original), then save it.
Then hopefully it'll save your login image as the same colour format as the original.
I believe mini maps and things had this problem, and I did it by doing this in paint, so hopefully it'll work for this too. It's been a long time since I did it, so I hope it works.