Just a couple observations from my experience;
- On the DMsG, I'm not a fan of PWYW. IMO it devalues your product (people download stuff just because they can get it free, even if they never open it). Charge at least a dollar, or more likely several.
- If you want them usable on VTT's, most currently suggest 10 pixels/foot (50 px/square), but I expect changes in the next year or two will push this recommendation to 20 px/ft in the next year. JPG is preferred unless you are using transparency (for doors or traps overlays). JPG quality can be anywhere between 40-70%.