Copy the function theme_uc_product_image from uc_product.module and paste it on the end of your template.php. If you don't have a template.php in your theme directory, create a new one just to hold this function.

Inside your template.php, rename theme_uc_product_image to phptemplate_uc_product_image. Now search for the following lines in your template.php:

Delete those lines and replace them by:

That should get you started. You will have to make more edits if you want to use a div or span instead of just a br, and you might want to make other changes to the HTML to get the look you want, but this is the basic procedure.