Ha, this is one I can answer, as I managed to do it!
'Prepend' means add to before the quoted text, so you would have
../images/<?php echo WADA... ; ?>
This answer assumes your Search, Details and Results pages etc. (produced by DataAssist) are in a folder called admin, and your images are in a folder called images. If they're in different folders, then amend the paths accordingly.
Go to the Property Inspector (usually at the bottom of the window - if it's not showing, then hit F7 in Dreamweaver). In Property Inspector there is a field entitled Src.
If you have already used Bindings to bind the productimage to the placeholder, Src should show something like <?php echo WADA... ; ?>. Type "../images/" in front of "<?php", and hit Return.
Hope this helps!