This is unfortunately a sad consequence of posting pictures on the internet, particularly Flickr.
http://www.google.com/search?hl=en&q...&aq=f&oq=&aqi=

Many, too many, people think a photo 'sharing' site means they can use pictures for whatever reason they want, where ever they want with no regards to copyright.

Best thing you can do is post small sizes (72dpi) and watermark every image you post. It won't stop it, but may make your picture less attractive to someone wanting to steal it for their own use.

The only other option to guarantee your pictures won't be stolen is to never post them online or email them to anyone, but we all know that isn't any fun.

Sorry to hear your pictures were stolen.