I scan it in as a black and white photo at 300dpi (I think) and maybe that's what the problem is but I'm not sure. It looks about right at this size which is fine because if I did it on huge it might overload my PC.

The PC's own the keep on the southern edge but it's new to them and was recently taken over while they were out. Next time we play I purpose for them to find the entrance to the cave that leads to the entrance beneath the keep. Of course, if they are alert enough they may also find the secret doorway leading into a forgot Dwarven underground.