The “blog avatar” doesn’t make a lot of sense to me. Users are already being asked to upload an avatar for them… Ok, this is something usual. But… chosing an avatar for a blog/site?
Here is a little suggestion:
Would it be possible to offer the user the choice between :
1) an uploaded avatar of his choice
2) a thumbnail preview image of his site? This preview image could be generated by a free service like thumbshots.com for example.
The wp-admin/options-general.php?page=blog-avatar page could look something like this:
This is your “blog” avatar. It will appear whenever your blog is listed (for example, on the front page of the site).
You can choose to display an auto thumbnail preview of your site or upload a custom picture of your choice:
[ ] Check this to use this thumbnail picture
Upload a picture of your choice:
[file input field here]
The thumbnail preview picture could even be set as “default” by the plugin.
I hope this makes sense.