I agree that higher resolution doesn't always = better image, but it does most of the time. I think you will be blown away by the image quality on AAX. I've had an account for several years now, and I don't remember what the process was, but I don't think it was onerous. I know one thing for sure, they don't like VPNs. You'll either have to turn it off, or white list their site to get through.
So as I'm typing this, I figured I would check their site right now to see if I could register for another account. After clicking the register button, it goes to a page that says "
creating a new account is not currently possible". Well that sucks! I guess they're limiting new memberships? I then searched Reddit and the first post I read
https://www.reddit.com/r/musichoarder/comments/lph9pb/an_update_to_the_rules_regarding_album_art/, a user claims the site owner sometimes ban entire countries... Wow. I have been using the site blissfully for at least four years now while apparently there is a major drama going on behind the scenes!
It would be nice if JRiver had the option to select a minimum resolution to show when using their cover art retrieval tool, but I don't think they keep every image uploaded. As I understand it, the images that are kept are based on which ones users download the most. So if you have a bunch of users who only want 300×300 images, that's what you're gonna get as options to choose from.
And finally, for image compression I use online software that does a phenomenal job,
https://resizing.app/. If the album image is a photo (versus an illustration), this site will reduce the file size from ~ 2mb to below 300 kB (for JPEG files anyway) without reducing the image dimensions. And does that without any visual difference between the images (at least the vast majority the time). If the album art is an illustration, I try and find the largest size possible… Which is usually 1500x1500, run it through the application and reduce the image dimensions to 1200x1200 or 1000x1000 if I have to, in order to reduce the file size below my threshold. My goal is to end up with a very large image but is still < 300 kB, but still looks very sharp and crisp.
If JRiver could be convinced to store all images uploaded, I would re-upload all my high-res images. Every time I check an image for an album cover I know I've uploaded, the majority the time it does not show up as an option a few months later. Presumably this is because users have opted to download lower resolution with smaller file size versions. I'm not sure I understand that, since file storage is so cheap, why wouldn't everyone choose the best quality image you could?
Oh well… Not much I can do about that. In any event, I hope all the blathering above as provided you some useful information.