You actually can, Kenneth, since transparent PNG format is supported. I've used this for purposes like adding Rain or Snow across the entire map, so it is a semi-transparent map tile with lines of falling snow; the marketplace also has the same idea for a toxic cloud. Very large PNG with transparency like that can cause a lot of lag on some systems though (lower end computers especially), and it has other disadvantages. One problem is token-selecting, if your monsters or characters have tokens -- will they be under or above the transparent graphic? If they're under the transparent tile, it's harder for GM to select the ones that are under. There may be reasons to try it, experimenting is good. If it fits your game purpose, I would advise that it may be better to use discrete objects on the token layer, rather than one complete transparent overlay. Secondly if it is a permanent overlay addition, it may be better to go into a graphics editor program, add the overlay, and save it to your background image as 1 non-transparent background image. (For example if you want to add a topographical map line study). What kind of map do you want to accomplish with this? I could see using a large overlay for something like a HUD or tactical / wargame / positions overlay, if you wanted to lay down whole scenarios at once and didn't need to move the individual X's and O's on their own.
