To make sure, that you are on the right place to ask your questions, what tagger application do you use, is it Mp3tag 2.49 or last version Mp3tag 2.49a?
Where resides the cover image?
Is it an extenal file anywhere on the disk or is it an image on the Clipboard?
What are the steps in detail of the operation you do with Mp3tag to load a cover into a file?
What problems with your music file do you expect or would you assume?
My version is 2.49a.
But I thinks that I found the problem, it seems a bug of win 7 because I tried to do the same operation in another pc whit win xp and it works fine
right click on file --> tag... -> add cover --> (select jpg file on another folder) --> save
Hmm, I can follow your workflow only for the first step ... "right click on file".
I cannot see the next step in the context menu, which you have named as "tag...".