At first, Ryu's image posts were showing up mostly blank, with no background color at all. However, there was some hidden text showing errors with the function used to calculate the average color for the image post backgrounds. Using that clue, I tracked it down to a host-specific PHP issue.
For Media Temple's Grid Service (and some other hosts), allow_url_fopen is disabled, which breaks that PHP function. Media Temple provided instructions on how to enable allow_url_fopen by modifying the php.ini file, which may come in handy for other hosts too. I've verified that this fixes the issue, and am now enjoying this gorgeous theme.
Hope this helps keep someone else!