VTTs typically use a common image file format like jpg or png. Usually the former since the files often compress better. For my games, I usually use a resolution of 100 pixels per square. That seems to get me the best results for speed of file transfer versus visual quality. Since everyone at the table has to download the map and all of the tokens, it becomes important to manage those file sizes. Some people may like larger resolutions, though. You might ask the fans what size they'd like to see.
GMs using a VTT frequently like to have many items on the map as individual tokens. These are usually saved in PNG format so they can have transparent pixels (allowing images that do not appear to be rectangular). Doors should almost certainly be tokens of this kind, as should anything the PCs or NPCs are likely to move, smash, or throw at one another.
Helpful hint for doors: Make the door image twice as wide as it needs to be, with the hinge in the center of the token. That way, a simple rotation can open or close it, without the GM having to reposition it.
You will probably want to make gridded and gridless versions available. Most VTTs can generate their own grids, and it's usually easier to line things up if you're not trying to align it precisely with a burned-in grid.
If you want to get really detailed, you could make some packages of prepared VTT scenario files with the maps already set up. I find that one of the most time-consuming parts of prepping a MapTool session is getting the visibility layer set up. This is an extra layer of information that tells the VTT where objects that block line-of-sight are. With that set up, the software will automatically handle fog-of-war and lighting effects based on the locations of the players' tokens. Very useful, but a bit of a pain to configure.