Wish my gfx artist was around, as he would be able to explain what he is looking for ten times better, but i'll explain it the best i can. Since pictures are positioned on screen by window, that means that no matter how big you make a picture, that it will always be moving based on the characters x and y position on the map. Let me try and give a really good example of where a fixed positioned picture would come in handy vs making it a character set, a tileset or a fog layer.
The most useful feature that pictures have vs fog layers, tilesets, charactersets, and anything else of that nature is the power to animate through multiple pictures without a given size limit, and also the power to make the image transparent or something along those lines. Character sets and tilsets have no where near the flexibility as a picture does. Fog layers are the closest thing that comes to a picture, but i'm already using the fog layer for lighting.
I guess what i'm asking is, is it in anyway possible to position a picture on the map instead of the screen window? Rpgmaker 2k3 had the ability to show pictures anywhere on the map, even if you weren't anywhere near it's coordinates if i remember correctly.