Go to settings, make sure that the field Picture is checked under Quick Access Frames.
Now open a song that doesn't have any picture tag in it. Try to edit the Picture field by selecting it and click the Edit... button or by double clicking where the picture should be displayed.
A text field is opened expecting some sort of text input. Nothing else. No way to attach a picture, or edit any of its properties.
The APIC field should be opened allowing me to edit the Picture frame, where I can import, export, from clipboard and even view the Picture.
I've only tested with MP3 files, I don't have other types of files to test this bug with. I also imagine that the type of file might not have a lot to do with this bug.
Also, if the song already has an APIC field, then double click on selecting EDIT does work.
I will check when I'm back at the computer. Probably it works if there is already a tag in the file. Drag and drop of pictures should always work so it should not be severe.
I have now added a fix to the devel branch (and will push it to master after more tests). If you don't mind building from source, you could test it.
Fixed in version 3.4.2.