I think technically
rangemap when used with the
<single> UI element (aka the "SingleValue" module) is setting a CSS class based on the
classField value, so I suppose you could add your own custom
.css and use that to swap out images. I'm not all that familiar with style sheets or web development myself, so I can't give you specifics. You may want to post this a new question about it. I'm pretty sure that some of UI gurus could give you more details on how do this.