Are you saying that the image is displayed but it is not a link, or are you saying that the image does not display at all? My comment below assumes the later.
I have done this before and it worked fine. Have you looked at the HTML source within the browser? You should see the < img > tag within the < a > tag. The issue is probably that the src path is incorrect; viewing the generated HTML is the best way to debug the problem.
When I use text, the following is produced, and I can see the text:
<a href='/portal/auth/portal/default/ao/ad?action=1&perform=backToSelectDB'> back to select db </a>
When I use the image tag, I get this:
<img src='/weborganiser/images/edit-undo.png' HEIGHT="22" WIDTH="22" BORDER="0" ALT="Back to database select" TITLE="Back to database select"/> <a href='/portal/auth/portal/default/ao/ad?action=1&perform=backToSelectDB'> <img src='/weborganiser/images/edit-undo.png' HEIGHT="22" WIDTH="22" BORDER="0" ALT="Back to database select" TITLE="Back to database select"/> </a>
No image or link is visable, no link shows up using the Firefox Web Developer extension, and highlighting the anchor tag.
I can't see anything wrong with the HTML. As an additional test I also used the image tag code before the anchor tag. The image displays fine outwith the anchor tags, but disappears within them.
The image shows up fine in the image input tag.
That is strange. When I do the same thing I get the image showing up with a blue outline (I overrode the style to get rid of the outline).
Try this. Change the image properties to lower case (e.g., height="22"). (In my code I did not provide a height or width.) Perhaps Firefox doesn't like uppercased attributes on an XHTML page.
The only other thing I can think of is that a style setting could cause the problem. Do an Edit CSS (since you have the Web Developer extension) and see if there is a style sheet other than portal_style.css.