Ah, okay.
It's not very usual to play D&D in a Mortheim-like world. But I think that fits pretty well.
Recently I played a RuneQuest Legends session in the g+ hangout with some friends. We used maptool, too, and that worked without problems.
I like this development of online role playing. But while playing I wished I had two monitors.

144 dpi would be a lot better, but 100 dpi is acceptable.
Wouldn't it be possible to paint it in higher resolution by dividing it into smaller parts?
What is the maximal file size your compi can handle?
Do you work in Photoshop?
Or is it a problem of handling the final file in maptool?