I'm not sure if you are trying to design based on hand painting or computer generated design from stock images but Lifehacker has some terrific guides on http://lifehacker.com/5781134/digital-painting-101-the-complete-guide?tag=nightschool
This. I can't stress this point enough. After working on web design for quite a while now, the first thing I'll teach newcomers is that you should make a clear distinction between your main HTML page and your CSS.