.img files are CloneCD files created as a result of dumping contents of a Cd into a file. Very similar to .iso files which are more popular for containing cd images.
Many tools, like virtual box allow mounting .iso files as a virtual cd/dvd. Ubuntu repositories provide a tool called ccd2iso for converting the .img to .iso.
just install ccd2iso from the repositories using
sudo apt-get install ccd2iso
Once installed, just launch it for converting
ccd2iso file.img file.iso
This will create the iso equivalent of the img file.
To mount the iso file normally in the linux environment, just type
mount -o loop -t iso9660 filename.iso /mnt/iso