Funny. I mostly registered here because I was amazed at the accuracy of the Advance maps on The Sonic Center :). I guess they were made by ripping the stages and the sprites straight from the ROMs; they're not credited, but considering Rolken recently ripped all the stages from Sonic 2, Sonic Chaos and Sonic Triple Trouble for the Game Gear on the Sonic Retro forum, he might have done those too.
At any rate, I gave up long ago on completing them because they were a real pain to do considering the size of the stages and the huge number of screenshots required to make even one (I think back then I averaged a stage every few weeks or so). Nowadays I'm trying to map the main Sonic games for the Genesis (Sonic 1, 2, 3 & Knuckles and CD) using the built-in Debug Mode which makes it a lot easier and thus feasible to actually finish them in my lifetime :)
Whoever did the Advance maps on this site, I'd like to contact him both to try and get full-size versions of them, and also to offer to do the backgrounds for them. I mostly map games because I like the graphics of them and not to provide help, so aesthetics is and has always been my goal, and making those maps look as good as they deserve would make me really happy.
Anyway, I don't know if I have the right to "advertise" here the game map forum where I hang most of the time, but search around on Google and you're bound to find it. I've done so far about 40% of the maps from Sonic The Hedgehog and Sonic CD, and I switch from one to the other whenever I grow bored.
Anyway, thanks for the interest and for the welcome. Hopefully, I can one day finish mapping an entire Sonic game, since it's a shame that nobody so far has done (well, with quality maps at least).