This isn't a security breach with Plogger, and it's not been hacked either, as your gallery is still intact.
Spammers have targeted your gallery and are most likely using an automated program to leave these comments on your images.
If you're using Plogger3, you can go to Options and check the box next to "Place New Comments into Moderation?" to have the comments held until you approve them. This would stop the spam from showing on your gallery pages, and you can just delete them.
The only way you can try to stop it completely that I'm aware of is to start using CAPTCHA with your comment form. There are a couple of threads in the forum with instructions on how to implement the CAPTCHA function with your theme:
The captcha system for Plogger will display an image with random text in it that the commenter has to manually enter into the form box next to the image before Plogger will accept their comment. Automated programs have a difficult time getting around this, as they are not able to read the text that is displayed.
Upgrading from version 2 to version 3 is relatively easy - here are the instructions from the main Plogger site: http://plogger.org/docs/upgrade/
Before attempting any type of upgrade, be sure to back up all of your Plogger files (including plog-config.php) as well as your images to your local computer. Also, make sure that you do a database backup via phpMyAdmin before upgrading. That way, if something did go wrong during the upgrade, you can quickly restore your gallery to the previous version if needed.