As much as i know, you once have to unlock bootloader with full wipe (this is required for flashing superSU which in turn is required for things like Bootunlocker.)
I have never tried this but does adb pull work for cache? you can get a copy of your cache with it. Also stock gallery stores thumbnails, these are not high quality and without root, they may only be option.
Device: Google Nexus 4, GTab 2 7.0, Galaxy Ace Plus, Galaxy note 2 n7100
ROM/OS: PA 4.2 BETA2, Stock Rooted, SlimBean (4.1), CM 10.1
Launcher: Nova Launcher Prime, Trebuchet 0.1.5, Smart launcher Pro, Nova Launcher Prime
Class of Sdcard: N/A, 16GB(class6) all,
Clocked: 384MHz~1512 MHz, 700MHz~1200MHz, 200~1008MHz, 700-1200MHz.
Past: Samsung Galaxy Apollo, SE w200i & w700i, Nokia 1100 & Communicator 9500, Motorola Atrix.