First of all thanks for the excellent plugin! It's quite challenging to find a decent Google Plus / Picasa photo embed plugin for WordPress but I think yours is definitely the best.
There are two features that I'd like to recommend:
1) Lazy loading of images could seriously speed up page loading times by only loading the images when those appear on the screen. http://www.appelsiini.net/projects/lazyload
2) When an album contains a large number of photos it'd be useful to only display the first cover photo and display the rest upon clicking on the cover photo. There could be an option for that.
That's everything that I can think of. By the way, I especially love the fact that I can select from a large number of lightboxes. For example currently PhotoSwipe is broken on my browser so I switched to ThickBox.
Keep it up!