Why doesn't my tileset show in the Editor?

[Kittah]

Perhaps a stupid question and I may be totally missing something, but I'm having problems getting my tileset to load in the Graal Level Editor. Basically, it just doesn't appear in the Editor.

The file is an 8-bit .png, first saved as .gif and then converted using the program included with Graal. I've added an NPC to set the tiles using addtiledef and the appropriate line has been added in offlinetiledefs.txt or whatever it is, but it just won't appear...

Any help, obvious or otherwise? I'm running Vista, if that changes things at all... I know that when I close the Level Editor I must do so through the task manager or it'll come up with some kind of runtime error... ugh.