For what it's worth, you can safely ignore "dpi" if you're not designing for print. All that matters to a VTT is the number of pixels--horizontal and vertical measurements--not how large the image will be on paper, which is the only place that dpi matters.
So for the purposes of this discussion, a more useful question would be "What is your working resolution?"
For some guidelines on mapping for VTTs, it may be useful to look over the dormouse's assessment of the various entries for the May mapping challenge: May Challenge Voting
Those comments relate specifically to making battlemat-scale maps.