You haven’t accidentally switched to the front (selfie) camera, have you?
(I’m on the old OS, 4.2.2., not sure if this changed in your newer version) When I launch my default camera app, there is the settings wheel in the bottom left corner. It leads to a black window with three tabs – choose the photo camera tab, and there you should be able to choose “image size” or “picture size”.
well well well, I missed an option called “reset settings”, in the camera app (it’s the last one on the too small dialog box - that’s my excuse ). Resetting the settings made many size options to appear : 1, 2, 3 Mpx, but also the QVGA one.
I really don’t know what setting I could have changed, but at least, my 8 Mpx camera is back.
Sorry for the bother, hope it could help someone else one day