It's hard to explain what I'd love to see, so I made a picture to kind of explain it. My ideal world would be that it would center any non "square" album art and fill in the dead space w/ a color from the album art, similar to how musicbee fills in the track list based on the album art. If it would be too much work, I get it.
Just my OCD talking! (and if this already exists and I'm just blind, I'm ready to learn)
I'll totally +1 this
Another thing to try if you're willing to do some work - along the lines of what phred is doing (Since I use artist view the images are conveniently saved in their own folder. Embedded artwork would be way more complicated)
Irfanview (free program BTW) has a canvas size edit function. One of the methods to resize the canvas is to pick a fixed aspect ratio - 1:1 would the choice (iview auto-centres the image as far as I can tell). You can pick what colour for the canvas - either using the typical colour palette, or by clicking on the image itself. If your lazy like me, then the just leave the canvas colour black and use the option to fill new space with blurred image (I used the full 199 blur). I tested this on a few images and it doesn't look too bad.
Now here's the best part… Iview has a batch processor. Once you decide on your canvas size options, just select the folder with your images and grab a beverage until its done. (images already at 1:1 are basically unchanged)
Now I should point out I haven't actually tried the batch processing myself - I only discovered it after reading this post, but I do have a pretty massive chunk of images that could be "fixed" myself. This is the simplest (read lazy) method I've found so far, so I may just make a backup of the image folder and give it a whirl