Some time you will find CD images with .img extensions. IMG files are raw-data copies of optical media and are primarily used to store CDs with odd properties,such as sectors which need to have read errors when read. Conversion to ISO format removes this information,as ISO format does not support this.
You can manipulate such images if you install ccd2iso package.
sudo aptitude install ccd2iso
You can convert img files to iso files as shown below.
ccd2iso file.img file.iso
IMG files almost always include a SUB file, which contains additional data for the disc format, and a CCD file, which is a plaintext configu-ration file describing the disc layout.ccd2iso does not make use of these files