There are two options:
1.) difficult but powerful: use the same method the RF guys use with xcss and so on and generating the images on the fly. Here I can't help and I don't think that anyone outside of the RF team can ;-) Search in the forum for discussions about that.
2.) simple and straight: copy and save the image you want in the browser. Use it as background image (with css) for the HTML element you want. I'm doing that for the rich tab panel because I don't like the predefined background of some styles.
Got it or need more information?