I had this exact thing happen to a photograph I had posted on Flickr. A magazine from the UK took my image without license, and published it in print, online, and in a DVD. Their distribution included many stores here in the US. Before posting the image, I had taken the time to register my copyright with the USCO, so that unlocks statutory damages, which are very important in copyright infringement cases. Without registering *before the infringement*, you don't get attorneys fees or court costs, and you only get actual damages. That makes pursuing an infringement case a loser if you don't have the copyright registered.
Having distributed physical copies into the US, I could have pretty easily used the US as the venue for a lawsuit, or even done so through a solicitor in the UK. However, lawyers are messy, and enforcing judgments across international borders is costly. The first thing I did was prepare an invoice stating the usage of the image, that there was no implicit license, that the images are copyrighted (which they are, whether you registered them or not), and how much I expected in licensing fees for the usage. I also added a multiplier for needing to issue the license retroactively and having discovered the infringement myself. After a bit of negotiation as to the licensing fee, the publication was very apologetic and wired me the agreed upon rate. Now they have a license, I got paid for the commercial use, and we parted amicably.
For more information, I highly recommend the ASMP website on copyright: http://asmp.org/tutorials/enforcing-your-rights.html
