If I'm honest, it seems like you're over-complicating the feature request!! It's absolutely not necessary to export the images to files. It's also not necessary to try and determine which image is which, for the purpose of my request. I'm only - simply - requesting that the resolution of the currently-displayed image is available as a new Information Field, which can be added to a column. Nothing more!
Let me show you my screenshot again:
Here you can see, that there are already Information Fields for the mime-type, image size, and cover type. It's certainly not necessary for Mp3tag to export the images to files, in order to calculate those three values, is it? Mp3tag simply grabs the mime-type, size, and cover type values of the default image, which is already displayed in the sidebar. If the MP3 file has more than one image, it's too bad - Mp3tag will still only show the values from the default image.
Therefore, by extension, Mp3tag can also show the image resolution of that SAME image. No need to export to a file, or do fancy calculations or anything else. Just take the SAME image that is already used to show the mime-type, size, and cover type columns, and grab the resolution of that SAME image. Very, very simple - right?
Do you get what I mean?
I think the things you are proposing are going way beyond what I (and the other users in this thread) are hoping for. I just want to keep it nice and simple! (So yeah, maybe your extra wishes should belong in another thread?? )