I searched for sites, though there weren't really suitable ones.
But maybe these ones will be helpful.
As far as I went through the pages, they've got only screencaps till Ep. 3, I guess.
Alcatraz (TV Show) Screencaps on Fanpop | Page 52
But there are also promotional photos, set photos and comic-con photos, you just have to go through the pages.
Alcatraz - Season 1 - New Promotional Photos HQ - Alcatraz (TV Show) Photo (27293872) - Fanpop
Another one is this here, just click through the pages.
New photos on this wiki - Alcatraz Wiki