1. Yes you can use only the thumbnail and full size image. We do also support a medium sized image as well.
Yes there is a setting that you can enable to autoresize the image to make it fit.
2. Can be done but would require some file edits Gallery.php files. The filename is randomly generated to avoid name conflicts.
3. For picture area tagging system. Example is found here http://www.smfhacks.com/storedemo/index.php?action=gallery;sa=view;id=2
Hover over the water there is a box in bottom part of it
The tags/keywords are not used in the name of the image. The filename is autogenearted to make it unique.
Gallery demo found at http://www.smfhacks.com/storedemo/index.php?action=gallery