Are you referring to an image of the CD contents, i.e., the CD was ripped to one large-ish file instead of to individual tracks, plus the CUE sheet that describes where each track starts as well as other info such as artist, track names, etc.? If so, then what you actually have is a single WAV or FLAC file that the B2 will play as one "song". It doesn't know what to do with the CUE sheet, but it can play a WAV or FLAC file.
If you're referring to a picture ("image") then it has to be in jpg format with the filename ending in .jpg.