The iTunes 12 Get Info dialog lacks right-click context menus (at least for now) but it still accepts most standard keyboard shortcuts and drag & drop actions.
Delete Artwork
On the artwork tab of Get Into, tap the image in question, use Backspace or Ctrl+X. Delete doesn't work.
Or, tap the image at the top left of Get Info, this time use Backspace or Delete, it is Ctrl+X that doesn't work.
Add Artwork
Select artwork, press Ctrl+C to copy, tap the center of the Artwork tab, press Ctrl+V to paste.
Or, select artwork, press Ctrl+C to copy, tap the small image area at the top left, press Ctrl+V to paste.
Or, you can drag & drop to, or sometimes from, the same locations.
You can also copy artwork from the Get Info dialog to Windows Explorer with the keyboard shortcuts.
If the source image is on a web page right-click and use Copy Image rather than copy. (With Chrome I get a one pixel wraparound so tend to go for a temporary local copy anyway - not sure whose bug this is, but it isn't new.)
If you have multiple images embedded drag the one you want to be the primary artwork to the leftmost position.
Replace/Remove All Artwork
The top left image box can be used to add or delete art from multiple tracks. In either case all existing artwork in all tracks is replaced/removed.
Old Style Get Info Dialog
Select some items, right-click, hold down shift, then click Get Info from the context menu. (Use alt/option on a Mac)
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