Do you know the person who sent you the file and do you trust the contents of the file.It could be a virus or malware and if you are not sure why you received it I would just delete it.
An IMG file is a file that can be a disc image file containing the entire contents of an entire disc. ie a copy of an operating system or similar.
It could also be an image file that is used by some camera systems.
As this is not a BT problem I would suggest that you do an Internet search on "what is an IMG file and you will find plenty information about what it is and how to open it.
