To attach images to products and catalog categories, you'll need to make sure you've installed and enabled the CCK, Imagefield, and Imagecache modules (and we recommend the Thickbox module as well). For Drupal 6 you will also need ImageAPI and either ImageAPI GD2 or ImageAPI ImageMagick, depending on the software available on your server. If you're not sure, choose ImageAPI GD2. These are all mentioned in the installation instructions as modules you should install.
The trick is then making your way to the Store administration panel once you've enabled the modules. There is a Store status table that logs various status messages related to your Ubercart installation. One of the rows in the table should include a link you must click that will cause Ubercart to automatically configure the various image settings. Click on that and you'll be ready to go!
This process is also covered in the basic product creation documentation.
If you're looking for image troubleshooting support, please refer to this FAQ.