Can you post the full text of the detection, you should find in in the avast Log Viewer, in the warning section, what you saw may have been only part of the path.
I'm not sure if avast would contemplate scanning a 54gig file, if it is in fact that large (I'm not saying that I don't believe you but that is huge), 5.4 gig sounds reasonable.
If your main partition/drive is clean then it may be best to do another image and save that, if that works delete the other image.
What imaging software did you use ?
Me, I exclude my image backups from being scanned, one because they are large, 2GB each (5 weekly copies saved for history) and because before I do an image I have scanned the system.