Grouping suppresses images during command line export

This one had me scratching my head this morning: If you import an image and then group it with another component, the image won't be included in any PNGs generated from the command line.

Specifically, I brought in a 16x16 PNG icon and grouped it with a label component, to create an icon-and-text-label element that needed to appear repeatedly, across several mockups. When the exports were generated from the command line, all the labels were showing in the PNGs, but the imported icon images were missing. When I tried un-grouping one of the image-label hybrids, sure enough, that image reappeared when the PNGs were regenerated.

Bear in mind, everything looks fine within Mockups when the BMML file is loaded -- this problem is confined to command line exports.