I rather doubt the detection on a backup image would be able to be uploaded to VT or any of the other multi-engine scanners as a hard disk image even when compressed is going to be massive.
I guess that the detection is only on the actual backup image file and not a file within it, is that correct (as you don't give the full details of the detection, file name and full path) ?
When I see WIN32:Hupigen detections on drive imaging software, I think FP as these massive, highly compressed files, seem to confound avast, you only need to search for WIN32:Hupigen in the forums to see they are invariably on backup image files and or pagefile.sys.
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I scan my system before doing my weekly image backup and exclude my backup images form being scanned (they are inert until you elect to open them or restore them), example g:\Backups\DriveImages\*.v21 this is the file type for my DriveImage backup files, replace the *.v21 with the * and your image file type.
That can be entered in the avastUI, Settings, Exclusions, if you accept the limited risk this may present, given that we don't actually know what file it is detecting, but as said if on a file inside the image backup then that is inert until you open/restore it. At that point if there was a truly infected file inside it would be detected by avast's file system shield as you restore the image (if restored whilst windows is running) or on a subsequent on-demand scan.