When I start to do this for my application, I usually make an image that is scalable, e.g. all parts of the gui have a certain location in the image, which I read out. Corners have a static size, but frame edges can be stretched (and thus are usually 1px in width). Perhaps you can use this idea to make a little (standalone) gui component editor in which you can load an image, select the object you wish to skin (e.g. button) and drag the scalable/static parts of the button like a puzzle in your image. Write output in a separate gui skin coordinate file or so, that can be loaded via your dll.

This way you can neatly have a single image containing the skin for all components, and by "unwrapping" all these gui components into your image create a skin coordinate file. You could obviously colors in the image without the need to create a new coordinate file, but when the scale needs to be different, you'll have to make a new coordinate file.