Can it be that you have not only ID3V2 but also APE-Tags in your files?
What does MP3Tag show you beside the cover-picture in the tag-panel? If it shows a name of the JPG-file and if you click on it it opens an external picture-viewer then you don't see the embedded picture but the jpg-file in the folder.
sorry is all OK, I haven't rescan the mp3. Thanks for your help
Last question, add the covers damages the mp3? I can remove and add a cover and, in future, remove the old one for add another in better quality, without damage my mp3?