Hi, would you consider adding option Stay on Top?
Another one is - would you add a possibility to open the Add Cover image browse window by just double-clicking on it?
And the third one - could the toolbar icons size be increased?
All these would come very handy...
How does your workflow look like so that you would find this function useful?
Yo do know that you can drag&drop an image file on the cover area in the tag panel to add that picture?
Well, d&d is of course nice and welcome, but adding one more way to load an image would be also fine. After loading music files by d&d, immediate and straight double-clicking on the image area would be quick and smooth. I hope nothing prevents to add such a feature, it would just widen the scope of ways how to load the image 
How would you get the cover for all the files that you just d&d'ed? If the file is in the same folder as the audio files, wouldn't another d&d be much quicker than navigating to that folder with the help of another window?
But if you already know the way with d&d you can bridge the time until such a feature is implemented.
You actually do not have to navigate there. As soon as you right-click on the image area and choose submenu Add Cover, the browse window will open by default in the same folder where the MP3 files and also the cover image are. What I'm suggesting is to allow the user to achieve this by also double-clicking on the image area. As simple as that. In my preferred style of work, this would be nice and productive ehancement. And those who would prefer not to use it, can just ignore such a feature...