How to upload multiple images to Ubercart product pages via FTP?
The way I did it in similar, but not exact circumstances, was to use Node export module. It has it's import option. On an import, it expects images to be already uploaded, so the steps are:


Create two products with couple of images manually
See where it's images went
Upload the rest of your images in the same place
Export your products to a file format you are comfortable with, like CSV / DSV
Add the rest of your products to that file, using your manually added ones as examples
Import products from file


My nodes weren't Ubercart products, but above procedure should work none the less.

Remember: Files uploaded will not be managed until and unless you import nodes to use them, so it's a good idea to turn off any "garbage cleaning" modules you may have to avoid their deletion.