I don't understand why there's any need for width matching or anyI don't speak for Mark, but is usually done because every member's window might be a different width, and modern design sensibilities abhor having variable blank space around a fixed size image. And a fixed size image is inevitably a screen hog on one's mobile device but a postage stamp on another's large-screen monitor.
Why not just let people upload images as they like, and display themThose are additional complications. It isn't "simple and easy to use" if you have to tell the user to pre-process the image into a certain pixel size. Many users aren't proficient with any image editing tools.
What might be nice is an uploader that allows the user to upload any image and see a "live" preview that allows the user to zoom and pan the image within a rectangle representing the cover image display. In other words, provide a simple size and crop tool. Maybe it needs both a "max" and a "min" rectangle: always crop down to the max, but never crop smaller than the min. Then users with a logo or other specific art can use the min rectangle to ensure that the important part of the image shows.
What I suggested was a lot simpler (for Mark, but maybe not for the user): a simple checkbox that's useful for users that have a logo or otherwise fully prepared image, but not too helpful for a user with a photo of which they'd like to use only a portion.
And note that none of this required the user to think about pixel sizes: the pixels would be resampled automatically as needed to fit the selected portion of the image into the display window.