How did they find MBknight?
Something with a USB stick, cat pictures and all. So they recovered deleted (but not overwritten) pics of cats on the USB stick used for the extortion. So? Where is the link from random cat pics to him? Nevermind it weren't his cats, but from a neighbor.
Were those cat pics online, and they found the same pic again?
/CSI mode on:
Did they extract the DNA from the pics, run it through the dog-poop-DNA database, found out which dog had a fight with the cat, then followed the dog via satellite right to MBknights home?
/CSI mode off
Ente
There may have been something in the photo metadata (e.g camera/phone serial number) which gave them clues.
Apparently the iPhone even embeds GPS location data in photos unless you disable it.