.IBG files are ImgBurn graph files. Basically, they contain information about how the burn went, such as length of time, amount of data written to the disc, etc. It also saves a general snapshot of the write graph.
These files are only usable with DVDInfoPro.
You can delete them, and prevent ImgBurn from saving more of them by going into Tools > Settings > Graph Data and uncheck "Capture Graph Data".