You can certainly use the 'map' window part for any sort of pictures. I use virtual tabletops like this more often than not. I prefer to display a blank colour or tabletop image and then drop handouts onto it as required (*).
You can't increase the space though for the player videos in Roll20, but one alternative would be to play using Roll20 in a Google Hangout, which gives you the option to switch that space between the Roll20 screen and a full image of the player currently speaking (while keeping smaller video windows of everyone active at the bottom of the screen).
(*) Incidentally, one really nice trick for this sort of play is to take a small map, rotate it slightly and add a drop shadow in a paint program, and then drop that onto a tabletop background. If you then use gaming beads images with a 3D feel as tokens, rather than portrait or overhead image tokens, this can support the feel of playing in abstract.
While a full map with FOW, a grid, and representational tokens can be very useful for tactical combat, I always feel that it detracts from the imagination involved. Players are focussed on the map and their token rather than their character and the shared 'reality' of the game.
Spyke