Ok, I am having this issue as well.

First off, I'm "fairly" adept with Drupal overall, as I've been using in on a variety of sites for about a year now, but first time checking out Ubercart (I very muhc like what I see so far too!)

I can upload images to a product, but NOT to a category.

Currently the installation I have running is in a dev environment setup per the user's guide on this site.

My files & /tmp "should" be correct, in that I can upload images to individual products.

If I can provide any other required info (versions etc) I'm here...

thanks...