This certainly sounds like a GOOM issue - Qt in N8 by default uses OpenVG paint engine to do hw-accelerated rendering onto a composition surface that takes a chunk from the 32MB of total graphics memory. And so does each one of your thumbnails (how many do you have loaded at once?). And, finally, reserving the camera requires a lot of graphics mem (typically, over half of what's available).
To restrict the graphics memory consumption, maybe try implementing a pooling mechanism for the thumbnails - i.e, only use as many QPixmap instances as there are images visible on the screen and reload them on-the-go. Should be relative easy as they're all the same size.
You can easily test if GOOM is the culprit - try switching to pure software rendering:
And see if it helps. Note that sw rendering obviously carries a performance hit. Note that currently you can only select the graphics system at application startup, switching to raster paint engine cannot be used as a fallback when encountering GOOM.
QApplication a(argc, argv);