Can't add artwork - bizarre iTunes behavior

Question

Okay - this is bizarre - I imported an album into iTunes 9 (downloaded from the Artist's website).  I added the artwork and changed the genre and number of tracks, but these changes only "stuck" to about 4 of the 19 tracks.  No matter what I did, I couldn't add the artwork to the 15 tracks that needed it.  And - interestingly - if I next/previoused through the tracks on the album, all of the values that I changed would revert to the original values the tracks had when they imported.  It was almost as if iTunes couldn't change the ID3 tags in the mp3 files.  (Well, I guess it really couldn't when it came down to it.)
I solved the problem by right-clicking all the tracks and selecting "Create MP3 verssion" - essentially duplicating the MP3 files.  I was then able to successfully get my changes to "stick" to the new files.  (And then I deleted the originals I imported.)
What is this strange strange behavior all about?

Answer 1

There are three common causes for this:
your media are in a format that doesn't accommodate embedded artwork - typically WAV files.  To embed artwork you'll need to convert to another format - Apple Lossless or AIFF if you want to preserve lossless quality, AAC or MP3 otherwise.
your media files are read only - to fix this, use Windows Explorer to find the folder that contains your files, right-click and select Properties.  On the General tab there's a check box labeled "Read-only" - if this is checked, or is grey (sometimes blue), click the box so that the flag is unchecked and the box is white.  Click OK, and then OK again when the "Apply changes to this folder, subfolders and files" option selected.  Now try adding the artwork again.
Windows permissions issues are preventing iTunes from updating your media files (there's some anecdotal evidence of a change in this behavior in iTunes 12).  See turingtest2's notes on Repair security permissions for iTunes for Windows for advice on fixing this, then try adding the artwork again.