A lot of different files use the .img extension. It could be a photo but if it's a large file then it is probably a CloneCD Image File. CloneCD is a well known CD copying software title. A CloneCD .IMG file should also be accompanied by a .CCD file and a .SUB file.
These files must also be in the same directory (same folder on same hard disk drive) and must have the same file name. For example a folder like C:Images would have to have IMAGE.CCD, IMAGE.IMG and IMAGE.SUB so that it can be burned to disc or mounted onto a virtual drive.
There is a variety of software titles that can make use of these image files. For example, Alcohol 120% or Daemon Tools can mount CloneCD Image Files onto virtual CD drives (so you don't need to burn to use them) or you can burn the image with CloneCD.