I'm afraid I don't understand your question. Where exactly is the problem? The gimp is only there for creating images. It has no connection with what you do with them afterwards.
If you have a question about how to use your images in html or css, then it would help to ask about that directly, in detail, with samples and examples. Although I don't personally have much experience with web design, I'm sure there are others out there who know more about it.
But then again, this is a general computing website, and you might get better quality help in a dedicated web coding forum.
There are also many html/css tutorials and discussions out there in cyberspace. I have seen recently, for example, that css allows for the use of sprite-style image slicing. Here's just one of many sites that discuss it: