I think I really am asking, is what exactly does the DPI setting do? IF I have a map with a defined envelope (X1,Y1,X2,Y2) being shown in a DIV of W width and H height, then the image returned is always the same size in pixels and the same spatial coverage.
Changing the DPI setting makes lines appear thicker (at higher DPIs) but that is about all the difference I can see.
Cheers
ACM