There are a variety of features that would be useful concening managing pictures:
Copy to clipboard*
Paste from clipboard*
Copy to file
Paste from file
Using the clipboard would make it easy for me to attach a picture from a web site. I would simply right click on the picture in my browser, select Copy to copy the picture to the clipboard, then use the proposed Paste from Clipboard option to paste the picture into the selected track.
Of course, you need a way to perform these operations on multiple tracks (since you typically want to add the album’s cover art into all of that album’s tracks).
You need to come up with a generalized method for applying a new value to the same tag in multiple selected tracks. This is a very important feature, especially with large libraries (I have 9,000+ titles in mine). I often need to apply a change to one of the MP3 tags for all of the titles within an album (album title, artist, year, etc). In some cases I may need to edit a tag for hundreds or even thousands of tags (for example, change the genre of a selected set of songs).
Your competitor, Musicmatch, does this by providing a checkbox next to each field in their Edit Track(s) dialog box. If you select more than one track to edit, the text box for each field is initially grayed out. If you check the checkbox next to a field, that field’s text box becomes active, and you can then edit or fill in the value you want to assign to all of the selected tracks. (If you edit a single track, the check boxes disappear and the fields’ text boxes are active.) They use the same appraoch for adding pictures to a set of tracks.
[This is by no means a plug for MusicMatch; your product has many great features that they don't have.]