Can't open .jpg files extracted from an .iso, what to do?

Judging from the error reported by the image viewer, I would say that the files are probably either corrupt or they're not really JPEG files.

What is the result of:

file the_file_name.jpg

The answer will likely be the actual type of the file.

Sometimes your image viewer has not installed the Eye Of Gnome (EOG) plugins. Go to the Software Centre then select 'plugins for eog' and then click 'install', then it should work.