Problem not solved at first. (Can't change mime-type for jpg-images). After restarting the computer, all behaves well. Problem solved!
Changed mime-type for jpg (open with GIMP) : ok, opens with GIMP
Changed mime-type for jpg (open with Shotwell) : ok, opens with Shotwell
Changed mime-type for jpg (open with Ristreto) : ok, opens with Ristreto
sudo apt-get install thunar/ utopic- proposed
Problem not solved at first. (Can't change mime-type for jpg-images). After restarting the computer, all behaves well. Problem solved!
Changed mime-type for jpg (open with GIMP) : ok, opens with GIMP
Changed mime-type for jpg (open with Shotwell) : ok, opens with Shotwell
Changed mime-type for jpg (open with Ristreto) : ok, opens with Ristreto