It uses the main post title, which places the same phrase over and over on the same page if that page has a lot of images. I think Google probably penalizes this, thinking it is keyword stuffing.
A suggestion would be to use the image's own "Alt" or "Title" tag, which is set when images are uploaded, instead. This would spread out keyphrases, and should be SEO friendly.
If someone has an idea of how to mod the plugin to do this ourselves, please let me know!