Adding artwork in iTunes 11?

I've been Googling for solutions on this issue for the last hour with no luck. None of the methods and workarounds I've read will work for me.

PC - Win 7 Ultimate - iTunes 11.0.2.26 (using to sync iPhone 4S and iPad2) - .jpg format images

I just want to be able to manually add artwork to my songs and albums which I have loaded myself (not downloaded through iTunes).

Selecting full album, if I choose "get info" and either drag/drop or copy and paste a cover image in the Artwork box and click "Ok" the image is not being retained by iTunes.

Anyone know what the heck is going on and how to get it to work the way it should?

Thank you in advance for any assistance!

Added (1). Messing around in the metadata, I found that iTunes will not write the artwork to the mp3 file if it is checked as "read only." So if you go into the file properties of each track through Windows and uncheck the read-only attribute, then you can manually add artwork through iTunes. Previous releases of iTunes must have automatically overidden the file attributes, explaining why it worked before but not now.

I don't think you can. But you can change it in Windows Media Player. Just go to the album art you're gonna put and right-click then copy. Next, go to WMP Library, find the album you're gonna change and right-click on it then paste album art.