If the art is called "cover.jpg" try renaming it to "folder.jpg". (Ah - maybe that should be "albumart.jpg" for droids... am checking details and will re-edit this post shortly. Looks like there is a confused standard out there)
If you have Windows 7 \ Vista \Win8 make use of the folder search.
* Attach the Android Phone to the PC.
* Use Windows Explorer to open the Android device and locate the Music folder.
* Open the music folder so you can see all the artist folders.
* Now use the Windows Search box at the top right of the Window to search for *.jpg
to locate all of the artwork in the music folders.
* Rename every JPG to folder.jpg
This is the trick I do on my Blackberry to get the artwork visible. My Music library on the PC has such a wide selection of ways the artwork is stored. I always find it quickest to just to the quick rename this way after copying new albums in.
(I also have to do the same with playlists as my Blackberry doesn't not recognise a *.m3u8 and needs then to be called *.m3u. The Windows Search is very useful for this.)
Ah... interesting... this may be a known Droid issue. I just spotted this thread via a search. Notice this is someone with an HTC who is spotting that their artwork is random as to whether it appears or not. But the moment he renames
the folder with the MP3s and artwork in it then the artwork springs to live. The read goes on to theorise that there is a database that needs updating.
There is probably a more "correct" way to force the library to refresh, but this does sound like an interesting lead. And a very easy one to test by just renaming a folder.
Okay... the more I search the more confused this gets. Seems the Android can be a little mad over the art. When the music is not moved in via Android own sync it seems to get fussy and confused. Logically it is that database that doesn't get updated.
Some people blame the MP3 ID3v2 tags. In those cases they have totally deleted the ID3v2 tag, saved the file, then restored that identical tag.
Some specifically blamed the comments in their ID3v2 tags and found they needed to be removed.
I would not be surprised if the messing with the tags was enough to force the Android to refresh that database of artwork.
Yet another solution pointed at the "albumthumbs" folder on the Droid. For one guy this was holding "bad" art and over-riding his newly selected artwork. He went into this folder and deleted the art for the "bad" albums and restarted the phone.
(This question really should be in a nice new thread as I think it is getting less about Winamp's cover art feature and more about the Droid... and no doubt other people have had similar issues. Have you tried searching through the Android section of this forum?)