It depends who it was. If it was a major publication. I would sue them! Sue them and hit them with a stick! But no major publication would ever do that cause that would be stupid.

If it were another "professional" artist showing off my work as though it were their's, I would contact them and tell them to remove all my work or I'll sue them! Sue them and hit them with a stick!

If it's just some kid on an online forum or something, I'd just send them an email asking them to stop doing that.

Everything becomes murkier if there is money involved. If another photographer is using my images to promote his/her own business without my permission, then maybe bringing in a lawyer is necessary. Otherwise, I wouldn't make a huge deal out of it. It's kinda flattering in a very rude sorta way.