You need to uncompress the image, you'll probably find a mule.exe file in it along with several other files. In dosbox you'll simply type mule at the prompt once you are in the proper folder.
Without further info I'm not sure I can help... ...but
http://en.wikipedia.org/wiki/IMG_%28file_format%29This will give you some basic knowledge.
Chances are good this was created with WinImage, a program that runs under windows, and probably will want to create some kind of disk or diskette, but may let you decompress to a folder on the hard drive or an external flash drive.
Goodluck!