As I've specifyed in the title: just gthumb image viewer stoped to display images with .jpg extension Furthermore images with another extentions (jpeg, bmp) are still displaying normally, I can view the mp3, mp4, wmv, avi icons and thumbnails, but jpg are not. Now I've tryed some straight methods:
  • Changes all possible combinations in gthumb preferences;
  • Restart gthumb;
  • Restart Ubuntu at whole (!)

It's no effect!
  • find ~ -name '*gthumb*'
  • Delete them all;
  • Restart gthumb;

It's without effect too!
And even radical method:
  • sudo apt-get purge gthumb;
  • sudo apt-get install gthumb;

It's unsuccessful!

A most mysterious in that there is normal gthumb behaviour in other users sessions at the same machine