My backgrounds are delicious at full size but they kill the site for mobile devices, because they're just too big.
The site background slider could be easily made way more mobile friendly if it offers a metabox to filter background images through something like src.sencha.io.
That one is an image proxy service that factors in screen resolution and serves the image downsized for the device, or fullsize if this is not necessary.
It seems like this would be an easy functionality to add to Site background slider, because it only requires prepending http://src.sencha.io/ to the image url.
So the image url to use the proxy looks like this:
I've hacked my copy of the plugin so it appends this to the images, but it would be great to add the feature, as a form on the Site Background Slider Options page, for URL prefixes. It could be left as a text field so other services can be used.
If you're interested in how it works, check out the full documentation.
I am not associated with that company, it's just the only thing I found that will crunch down those background images to mobile size and serve them up on demand. It doesn't reduce the images on desktop browsers, they get the prefix too but they are served full-size.
The sencha.io plugin for WP does not affect the background images on SBS. that's why i had to try the hack.