Hello, I run a site called Album Art Exchange (http://www.albumartexchange.com/) and am a bit dismayed at our exclusion from this list and your search workflow. After all, it would have been the #2 result when you typed "album art" into Google.
We were started 4 years ago as an alternative to the low quality which iTunes was pumping out as album art and the heavily compressed 500x500 images that Amazon had (and still does) as well as the garbage posted as album art on many of the other sites. We are up to almost 190,000 covers now nearly all of which are very high quality and the majority are user scanned and submitted. They are all a minimum resolution of 600x with most also available at much higher resolutions. Quality and resolution are not enough though and we also focus on accuracy.
I say this not because it is my site but: you will not find better artwork at ANY other album art web site and especially not any that you have listed here as your workflow. You don't have to believe me though, pick an album and compare images of it from AAE to any of these other sites. Not being one of "those sites" is exactly our reason for existence and our success! Pardon my hubris but seriously, take a look.
Google Images: You should that neither AAE or ACDC (AllCDCovers, the only other legit album art site, IMO) allow bots into their image so you will not see those images there.
Album Art Downloader: For various reasons, Album Art Downloader does not have access to my AAE, ACDC or several other sites. Use this and you are missing out on the good stuff.
Amazon: You can upload your finest quality image and their back end will compress it by a factor of .5 and shrink it to 500x500. I know, I have several on there.
mega-search.net: I'm sorry but, Seriously? I just checked it out again for the first time in a long while and AVG was going bonkers at the pop-ups. Many of these sites it checks lift their content from ACDC. 450 other sites,I don't think so... I'd bet $450 that there aren't even a quarter of that number of album art sites out there. Believe me, as a site owner, I look several times a month at what is out there.
CD images: We were just talking about this in our forums the other day. What legitimate use is there for an image of the CD? The only thing that I can think of is printing counterfeit CDs so, we do not have any of these.