First off, this plugin is fantastic. I've tried a ton of different Flickr galleries but none of them really fit what I was looking for until I found Alpine PhotoTile. Great job!
Anyway, I think I ran into a bug. When embedding pictures without an API key applied in the settings, 800px images are pulled from Flickr as expected; however, after applying an API key, the plugin seems to be pulling full sized images instead of limiting the images to the 800px size I set in the shortcode. As you would expect, this makes the page the gallery is displayed on extremely heavy and, on less powerful machines, basically unusable. If you'd like to see the problem, you can take a look at this page. It may also be worth noting that I am using a Flickr Pro account.
Thanks for your help!