I spent several hours banging my head up against the wall looking for a solution to the missing images on the Ubercart catalog and the checkout cart section. I searched and I searched and on google I found scores of other frustrated developers who had run into the same problem, but I didn’t find any solution posted. It turns out that the actual solution is very simple, but downright unintuitive.
Basically what you need to do to make this work is go to the edit page for the product content type (admin/content/node-type/product) and at the bottom, got to the field marked Ubercart product settings. Under “Product image field” choose the image field that you want the cart to use.
Now don’t ask me why the makers of Ubercart decided to put that setting in the content type form. I could think of a dozen other places where it would be easier to find, but hey, at least we found the solution.