besides the wayback machine ( archive.org) there is also domain tools which also archive sites and is more difficult to get removed.. They could also have a copy of the code of your site from when the infringement occurred, but this is highly unlikely, because if they had the code and looked at the code, they would actually realize that many of the letters they send out, have zero merit, as many of them are hot linked images.. Usually they only have a screen capture, which would most likely not stand up in court. They have to prove their case, and yes screen captures are easily eidtted/ changed, and screen shots also don't tell us if the image actually resided on the servers in question.