Try this first:
Go into Gallery Settings, select your Gallery and click Edit. Set "Image caption style:" to "Caption (thumbs & captions link to WP Attachment Page)". Save it. Then view an image. If this works, then you should be able to add comments just like that.
I believe there may be a bug in the way WP handles adding attachments to Pages, so if your gallery is on a Page (as opposed to a Post), it might not work. (I've logged a ticket with WP to get this straightened out...hopefully.)
In that case, if you buy the membership, you get PhotoSmash Extend, and you should be able to configure it to automatically create a separate post for each image that is uploaded via the front-end of your blog (Photo Manager uploads work a little differently since they use the WP Media Uploader). These posts are just like any other post and can get comments.
PhotoSmash Extend should work on your system if you're able to run the standard PhotoSmash plugin on it. So, I'd recommend making sure that works first.
Sorry I didn't give you a simple yes or no, but I figure this is better than a Maybe :-)
Cheers,
Byron