The best technique for this situation is CSS image replacement. Basically you have the link as a normal text link, but using CSS you hide the text and replace it with an image. There are several techniques, and I'm not too clued up on the specifics (search Google for "CSS Image Replacement Technique" or something similar), but I'd do it something like this:
Code:
css
a.imglink {
background: url("yourimage.jpg");
// some form of text hiding, whatever comes recommended - maybe negative indentation?
}
html
<a href="wherever" class="imglink">Anchor Text</a>
Now you may think that this is subversive, and might incur SE penalties, but as long as the image is representative of the anchor text, then you'll be ok. As a general safeguard, turn CSS off and view the page - if it doesn't look dodgy like this then it's fine. Obviously don't fill the anchor text with loads of keywords that the image doesn't represent - this is SE manipulation and will deservedly be penalised if you are caught.