I thought I had posted this suggestion several days to a week ago, but I cannot find any reference to it.
I would like the option to place album art in mp3tag without modifying the aspect ratio of the original image. For images that are wider than they are tall, this might require adding equal height white background "bars" above and below the image. This should not have to be done outside of mp3tag before placing the cover art.
This feature would allow images to be placed that retain their original aspect ratio, avoiding the unattractive distortion of the image along the horizontal axis. It would also prevent distorting copyrighted art used in the cover. Note that some on-line retailers such as CDBaby and Amazon appear to require square images in their store presentation. To meet this apparent requirement without distorting the cover art would be a very good feature.
In conjunction with this suggestion, I would like to see an optional setting to limit the maximum pixel dimension along the longest axis, typically the horizontal axis, in order to keep the sizes of the resulting files as small as practicable while maintaining desirable resolution. In my case, I think it wise to limit the image size in the horizontal direction to something like, say, 800 pixels. This would require downsizing the image when it was larger than the desired setting along the longest axis.
I admit, that collecting all the data in the right format is a problem. It takes time and a fair number of steps in a workflow.
Some of the problems arise from very specific local conditions. And these conditions then require local solutions.
Isn't scaling a problem of the target application?
In MP3tag you can set the display of covers to "keep correct ratio" (right-click on a displayed image - this is the bottom option in the context menu, I think).
So MP3tag can show the covers just as they are, if you say so.
MP3tag only modifies the tags of files. MP3tag does not modify the audio part or other complex object containers like graphics.
Mp3tag imports the pictures "as is" - faithfully along the GIGO (garbage in, garbage out) principle.
But you can export the graphics, modify them with a tool as powerful as you prefer it, and then re-import the modified graphics.
Modifiying graphics can require functions as powerful as offered by Photoshop - which is already available - and it costs you dearly. Just to get the proportions right: MP3tag is freeware, adding a (powerful) graphics editor ... might overstretch the bow somehow.
Anyway, I doubt that any graphics editing functions will ever be added to MP3tag. Or do you know whether Photoshop can tag mp3s?
But this is just my opinion and my way of reasoning.