I just tested what you mentioned, because honestly I thought there was a chance I was only confused by the lack of an explicit option for what I'm describing that I overlooked that it was doing it already by default. But it does not, so here is what I am describing, using an exaggerated landscape image scenario for clarity:
I have an MP3 whose cover art is 2000x1125 pixels.
I want square album art with a max size of 1500x1500
I set max size to 1500
--with crop on, the result is a 1125x1125 image that is square, but has lost part of the image
--with crop off, the result is a 1500x843 image that is not square, just scaled down
What I want is an image that is 1500x1500 but that doesn't lose any part of the image. (edit: 1125x1125 isn't a problem, the goal is a squared image without cropping, but since the max size box is there I'm including that aspect though not a concern I have) The image will be stretched, but it is complete. Normally they aren't in wide landscape format, so the distortion isn't that noticeable. But a cropped image can be noticeable depending on the image being cropped. Some albums don't have square art, like if it was released in digipak format. This type of resize seems standard in any image viewer, I just have no idea how difficult it would be to port into your app. It's usually a check box to pin the 2nd dimension or scale independently, which is what I meant my "preserve aspect ratio".
I hope that helps.