We've moved the discussion of posting attachments out of the "Step by Step - How to Contribute a Learning Object to VIPEr" document to a forum because the community uses many different structure visualization programs. This post contains some general comments. We haven't figured out the best way to do things yet so any suggestions that you have will be valuable to the entire community.
There are a large number of file formats that can be used to post static structural images. Choose your favorite but keep in mind that you want it to be easily read by the largest possible number of users. In other words, don't choose an obscure file type.