Hi - i'm enjoying the plugin.
I'd like to use it to serve backgrounds for the mobile version of the site too, but the fullscreen images take too long to load and are inappropriately large for such a small screen.
It seems like if site background slider pulled tagged images from the WP Media Library, it could somehow take advantage of the various sizes in the media library to serve the appropriate size image based on the screen (body) dimension.
In any case, it seems that Site Background Slider would benefit from being able to retreive tagged images from the media library, and not to be dependent on a designated folder.
However, maybe you tried it and that query was too slow. I don't know!