You're welcome Omri
1. You can submit all three of course, just a square to show grid dimension is enough for me as a GM, although grid lines can be a part of the drawing and/or may be helpful depending of the map (light lines over a dark background for example)
2. Usually 70-75% jpeg quality is a good compromise. for an encounter map loaded in a middle of a game session, keeping up the map around or under 1mb is good I think, the loading has to be fast enough to avoid loosing player's attention. Maintaining a good tempo, having a fair playing time between players without too much down time is important in a (virtual or not) tabletop RPG.
3. As this is a background map which will be loaded once in the beginning of the session and then keeped in players computer cache, rule #2 is less important here. I think around 2mb would be OK, here 75% quality again seems nice.