MediaFrame can display a single image or (as a memory-saving measure) it can start off displaying a small preview image and then switch to the full-size image if the user zooms in.  

It used to be obvious which we were doing: if we had two separate blobs, then one was the preview and one was the full-size image and _displayImage() would know whether it was displaying a preview image or not.

But now with jpegs, we might just have one blob, but still want to start off decoding it at preview size.  So the one blob serves as both the preview and the full-size image.  This is why I introduced the new argument.  If true, it means that we want _displayImage to display the preview.  If false, it means that we don't have a preview, or the full-size image is small enough that we don't need a preview.