I've always considered myself a bit of a neat freak when it comes to music stored in my iTunes library although not so much for other programs, so having the corresponding album art is for me, almost an obligation to maintain the library consistency. So when I go to order my new music and place the covers of each of the aforementioned albums, I find that the drag and drop option is no longer available.
I still do not understand how it is possible that Apple, having improved the overall design of the application from my point of view, have these small details that cloud the final result. However, the option is still there, but now we could say that it is hidden.
When opening iTunes 12 and selecting the chosen album, it would be enough to click with the right button (CMD + Click) to open context menu and select »Obtain information«, but if you look now only the loading of images from a specific location is available, so the process becomes more tedious, having to search for each folder instead of having the images ready and dragging and dropping each of them.
However, the traditional way with the classic menu is also available as mentioned above, only to activate it we will have to mark the exact same album but this time with »ALT» pressed, we will select the option to obtain information so that it is executed but with the old menu where this time we will be able to drag and drop the image that we want to our liking.
Sometimes the change of look in the interface also involves small changes in the options and not always for the better. In any case, with this little trick you will be able to handle the information of your musical collection as before.