Hi. A little confused, I even googled "audio image file". Anyways, your "audio image" file does have a *.wav extension doesn't it? You aren't talking about an iso file? You keep using the term "image". A wav file regrouping all the tracks is still a wav file, just a big wav file; or it isn't a wav file anymore?
If each file is a wav file with a wav extension, what I understand is that it doesn't always fail. One reason conversions fail has only to do with Windows limitations ... I suppose you have looked at the total file length which, including the path must be under 256 characters.
The only problems I have had with conversion was from un-mapped network files from classical music -- file length was always the issue for me.
Just out of curiosity, couldn't you just unclick "delete after converting", convert all files to flac and simply erase or archive all your wavs in one fell swoop?