Picard can be annoying when trying to add cover art, as there is no way to start a file browser dialog. In contrast, Picard does this when adding music to be scanned and pretty much anything else that you need to get into the filesystem for. Because of this, I feel that a "Browse Files" button or right click option should be added for the cover art section.

This can also be frustrating for users who use file browsers that do not support drag and drop or do not use file browsers at all in a custom desktop environment. I have personally resorted to using a web browser to drag and drop album art files from my computer.