Welcome aboard. I'm glad you're finding Ubercart to be useful!

A few quick responses...

1. The automatic image support offered by Ubercart depends on having a few modules enabled when you install Ubercart. If you enable them after the fact, you can go to the store status page at /admin/store and find a link to enable image support (inc. thumbnails + Thickbox displays). For more information, refer to the installation instructions.

2. As I think was mentioned, you have to create options for your attributes. Consider it this way... an attribute is some customer configurable aspect of a product, like t-shirt size. Options are the various choices they have, small/medium/large/etc. You create attribute/option sets independently of products through the attribute administration forms. Then you add attributes and options to products through the appropriate tabs on the product node's edit form. We believe this system provides maximum per-product flexibility while still allowing for some defaults to be set to speed up the product creation process. We left it so that attributes without options would be textfields to deal with special situations like engravings where you want the product to be associated with a special, custom message.

3. I'm not totally sure about what you're referring to for file attachments. Do you just want to attach files to nodes for display or are you wanting to sell file downloads? The former simply requires you to enable the upload module and ensure that the node settings for your product node types allow uploads. (Alternately you could use CCK and the filefield module.) The latter would require some extra configuration, instructions for which are sprinkled throughout the user's guide and forums.

Hope this helps! Good luck with the theming, too. I loved the mock-up, but we're no good at graphic design ourselves.