On our WP multisite network of sites using WPMUDEV's Ultimate Branding plugin Login Image replacement module, when Jetpack Photon is activated, then the Login image displayed on the login page gets resized~squished to about the size of the default WordPress logo image, rather than the original image size.
WPMUDV suggests: "...it seems that jetpack overwrites the CSS for the login page, and instead assigns the background-size as 80px instead of the zie of the actual image."
I have also confirmed that when Jetpack Photon is activated that it also breaks the thumbnail display of the plugin Special Recent Posts Pro
Both issues have also been reported to both plugin authors.
Unfortunately, we will be unable to use Photon until these issues are resolved.