That's not possible, however, with the new hexidecimal encoding, this shouldn't be a big problem because even large png files open quickly, so no harm done.
I really like that idea. I rarely use the keyboard navigation for the sidebar; I just don't see a need since I have the GoTo anything. I use Tito's sidebar enhancements to add some functionality to the sidebar, but that still involves mouse navigation, so I usually just keep the sidebar hidden.
This is possible if you use Selection > Expand to Scope. Just expand the selection until you have what you want to fold then press "Fold." Alternatively, you can Select by tag, indentation, or brackets.