I created another blog in the network and enabled the Image Store theme (in addition to the other network-enabled plugins) and sure enough, it worked fine.
The new test blog only had Image Store and the network enabled plugins and the 2013 theme. When I replicated these settings on the photography blog (only Image Store and 2013 theme), Image Store still wouldn’t upload properly.
When I replicated the photography blog’s settings (plugins and Tripod theme) on the test blog, the text blog also worked.
So it’s not a plugin or theme issue, apparently.
What sort of log files would help? Apache? PHP? WP? On a cPanel/WHM setup, where would I find the logs that would help?