IMG

IMG is the data container used for games such as Shin Megami Tensei III: Nocturne, Digital Devil Saga: Avatar Tuner, Digital Devil Saga: Avatar Tuner 2, Devil Summoner: Raidou Kuzunoha vs. The Soulless Army, Devil Summoner: Raidou Kuzunoha vs. King Abaddon.

PackTools
For unpacking Nocturne's and some other games' IMG files you can use TGE's Commandline tool DDS3Pack, which comes with PackTools. To unpack the DDS3 you will need to unpack the DDS3.DDT and DDS3.IMG from your game's ISO first. For this you can use programs like 7-Zip After you unpacked your files, open Command Prompt in the folder with DDS3Pack and your files and run the following command: If you had both DDS3.DDT and DDS3.IMG in the DDS3Pack's folder, then the command should unpack DDS3 content to the folder named "DDS3_unpacked"