Background images/videos should scale up/down irregardless of the screensize tag. The screensize tag is only useful for the infotext, shapes and windowed images (meaning the ones that doesn't take the entire screen, like game icons). If the size matches the selected resolution, then, no scaling is necessary. Note that while an 1920x1080 image may look good, it takes longer to load them.
720p and 1081i have the same aspect ratio, so any skin designed for each should look good on any of them. -that is if I can make smart scaling perfect
. But for all intesive purposes, if you don't want to do any automatic scaling and want it to look exactly that way you like it to, then, skinners should create an "optimized" version.
I'll add some settings to enable these options without having to go to the dash, experimental on my part, of course, and will take the need to press those buttons in future releases. "Back" will be the safeboot, meaning prevents switching to any of the higher res mode.