Chas.....image 'quality' need not be dependent on colour depth at all.
There is no reason why a bootskin image in 4 bit cannot be clever and effective....it's just that 'stolen' images of Britney's boobs in 24 bit will not adapt well to 4 bit which is why the section is moderated for such.
To do a 'half decent' job within a skin limitations....be it 4bit colour or fixed image placement a-la WA 2x requires a little 'talent' on the part of the artist/skinner, and not some 'twit' grabbing the first image he/she sees and claiming it's a bootskin.
What I dislike is those people who insist on misrepresenting their bootskins by providing full-colour previews in an attempt to fool the downloader into thinking they are good when infact they are not.
Sure the image is a jpg and therefore 16 million colours...but when it's reduced to 16 and gets seriously degraded it means they cheat, and that's indefensible....and the sign of a poor talent.
Winamp 'skinmaker' skins....aka 'photoskins' are the same thing....some weedy skin application tool fakes it for the newbie and we get Britney's dumb head on a skin for the umpteenth time.  Eventually they will learn to actually create the images from scratch [hopefully] cos they're not accepted here.
The difference between 'pushing limits' and not is the difference between 'Antares' and some dopey default Luna recoloured to purple/whatever and uploaded as if it is something 'new'....