you can get to the tutorials directly through the Gas 13 Site: http://gas13.ru/
heres another site with some nice tutorial to have a look at: http://www.spriteart.com/
Also there is nothing better than looking at ripped sprites from games, just do a search in Google for game sprites etc…
Heres a site with some nice rips on it: http://sprites.fireball20xl.com/
Metal Slug Rocks alright – I got the gif from here: http://www.persian-designers.com/forum/viewtopic.php?p=1565&sid=1d8002b306f0b6fe367ae4e9a56166d6
Metal slug recently got released on Mobile – Looks pretty cool. they kept it pretty faithful to the arcade version.