Oh, you can Google image search for higher-resolution images, and sometimes that's still the best you'll find - especially for the more obscure crap we love. But even then, it's a crapshoot with oversaturated images, terrible scan jobs, etc.
When I discovered Album Art Exchange, it was like another lil' slice of heaven for me. My OCD with regards to sound quality meshed perfectly with my OCD for good, high-resolution images to go with the tunes. There are some hardcore OBSESSIVE people over there who produce some absolutely amazing cleanup jobs of all sorts of record and CD covers. It's also seeded by folks who have direct access to the labels' art archives (for online use), and even then what these folks at AAX can do with a 30-year-old LP sleeve is often better than the labels' own high-resolution copy.
I've been an on-and-off contributor to AAX for a little more than a year. The problem was that until recently I just wasn't up there yet with the Photoshop skills to really get the best results from my scanner; what I thought was good often ended up too obviously 'shopped in retrospect. The other issue was that my 2008-vintage PC under Windows 7 would just flat out choke when trying to scan at anything higher than 600dpi - which didn't leave a lot of spare pixels when doing that hardcore editing most sleeve scans need.
Having recently switched to the Mac (mini; love this damn little thing after having been exclusively PC for the past 18 years), I now have the horsepower to suck down all the 1200dpi pixels my scanner can spit out. And I also spent some hardcore time reading threads at AAX regarding best practices for artwork cleanup. Over the past few days (while recovering from audio engineering burnout) I've added a shit ton of high-resolution images to the AAX archive, as I slowly go through and re-scan stuff I'd put up before or add things that aren't on AAX already. I have a ridiculous pile of stuff to scan that's not there as of yet, or that's only there in lower quality (back when the site was focused on 600x600 images). So keep watching AAX as I add my pile - and better yet, feed it yourself!
Here's a direct link to my stuff there. Here's a direct link to our blog pal 50 Pound Note's stuff there.