Thanks for the feedback. I've taken a lot of them to heart, and have included fixes for them in version 2.1 (which is currently being reviewed).
Snapper 2.1 will allow users to snap an entire webpage, without having to specify any dimensions. Users will be able to have better control over snapping webpages that contain frames (snapping the entire contents of a single frame, or even snapping an area that spans several frames).
Also, I have designated the PNG image format as the default for snapped images, although this setting can be modified by users in the "Options" window (to choose between PNG or JPG as default).
I will try to add boundaries in a future release, as well as a toolbar icon. If you have any other suggestions, or feedback, please post them here (or drop me an email at the address shown in the listing).