like 2of3 said its really very simple.. you put the margins in between whatever u want stretched.... or tiled and thats it.
As far as image size, that can be a factor.. if you import an image that is smaller than the one already being used in your template skin this can cause the existing margins to cross...which is a no-no and will need fixed, to make sure... Zero 0 out all the margins settings and start from scratch.
Aas image size goes, there are some areas with restrictions on size, if in doubt open up one of stardocks skins or a master skin and compare your image size to the image size they are using. if way off, use their dimensions or something close to it. Examining existing skins is also the best way to learn whatever it is.. you don't know.
Typically you can set the margins for the image at 4,4,4,4 and go from there, depending on how big the borders or corners are.. The margins will always have to be larger than the radius of the corners or depth of border
If you are just beggining I'd advise staying away from tiling and textures and start with plainer, simple images.
