OK, a bit further down the road, but still not clear.
Camera ICS creates a folder at /mnt/sdcard/DCIM/.thumbnails
Something else must create that folder too, because I had one before installing Camera ICS.
Doing several tests involved clearing appdata and cache, stopping, uninstalling, reboot, install other app, reboot.
If the .thumbnails folder is deleted Camera ICS will re-create it, and the cache file.
Huawei factory camera will not. Camera FV-5 will not.
If the folder exists and is empty, both Huawei factory camera and Camera FV-5 will create inside it the cache file thumbsdata3-nnnnn
in my case this seems to be around 500MB. I have a 32G sdcard and quite a few movie files and clips, a coupla dozen pdf files in a documents folder, and about 50 jpeg files in a maps folder. I copied the thumbsdata3 file to my PC, and it seems to be mostly empty. Running the strings comand gives:
What now? I've fired off a Bug Report to moblynx, but they may decide it's Huawei's fault.
Workaround, delete the .thumbnails folder and don't use cameras that use it.
And I'm glad I've got a Real Camera to take real pictures