One of the things I like about the system is you can add your own CSS styles file, and it comes last in the generation of the style on the page. So I have a running file of customizations in my file orfolio.css, in my case.
I use Firebug or Stylebot to view the element, then copy the style element from the netweb.css (or wherever) into orfolio.css, tweak the name from .nt-prompt to .orfolio-nt.prompt, for example, and customize it to my needs, then change the field or button's css element to my own. That would include adding an image to a button, of course.