That is possible as well!
I actual did it with an image, as well as text, within one of my recent email campaigns:
<a href="http://www.example.com/example?query=<span class="eloquaemail">ZZ___SFDC_ContactID1</span>"><img style="width:660px;height:137px;" src="http://www.example.com/example.jpg" align="middle"></a>
The trick is you can't enter the link within the link editor on e10 -> you have to actually type it out within the source, so if it is a textbox - right click it and hit edit-source. If this helps, let me know.
Regards,
Aziz nana