Ever upload your photo into a Google search to see where they appear on the internet? Amazing where my photos pop up. And flattering. And annoying.
Usually, I just think it's cool someone liked my stuff enough to use it.
Besides, there isn't much anyone can do about it. If it pisses you off, email them, maybe threaten to sue, but unless there is some real money being made off a photo, how hard are you really going to fight them? I've been through that and 99.9999% of the time this sort of litigation ends up costing more than it is worth.
People ask me to use photos all the time, and unless I hate what they are about, I let them use it for credit or maybe some money or barter. This band "Wrong" asked me to use a shot and I like their music, so why not? Kinda cool having my work on a record cover. Never would have found my pictures at all if I didn't post online.
http://wrongblackmetal.bandcamp.com/ Takeaway: Photo "jacking" is one of the many prices of technology. Fuggit... Personally, I would rather share my work and have it get stolen sometimes than hide it. Those are your choices these days.