It could just be a flaw in your phone or OS.... I'd suggest switching to a different OS and if that doesn't work, take your phone in for repair or replacement?
normally i'd say it was the SD card, sounds like an SD card, but your device not having one seems to negate that as a possibility. logic would say that memory is the issue, whether it be internal or external, but just sitting for a few hours won't change that enough to make the difference, at least not in my experience. a battery pull could, reboot of the phone might as well. what is your file free memory at? i'd start with deleting your web cache, call log, unnecessary text messages & apps, etc. BBs don't have much for memory on them and it's pretty much mandatory to have an SD card if you're going to store any videos, music, pictures, and such.
I just thought of this.... my 9700 came with a 2gb sd card inside already and I switched to my 8gb... have you checked to see if there is one in there?? As obvious as it might sound?
yeah i've had this happen, usually only when the photo album is already open, it doesn't refresh to show new pics sometimes. simply close the photo album app completey, and reopen it. this has always fixed it for me.