There are several aerial imagery links you can use, which each take imagery from different sources:
Code:
http://www.gcmods.de/toquad.php?x=~x&y=~y&z=~z - Bing imagery
http://www.gcmods.de/toquad.php?x=~x&y=~y&z=~z&service=google - Google imagery
http://b.tile.openstreetmap.org/~z/~x/~y.png - Streetmap imagery
Each of these is by default only available in 256px. You can get the bing imagery in higher resolutions however, but you need an API code. To get one, go to the
Bing Maps Dev Centre, sign in and then go to My Account -> My Keys and get yourself a Bsic/Not-for-profit key (I got mine at the beginning of 2015, so I can't quite remember how that process works). Once you have your API key, you'll need to use this link instead:
Code:
http://www.gcmods.de/toquad.php?x=~x&y=~y&z=~z&apicode=[YourAPICode]
You can then download imagery up to 2048px, but I wouldn't recommend more than 1024 as 2048 takes an unbearably long time to download for not much of an improvement (it could be worth it for high-detail city areas though).