Bonehead said:Theoretically no. Realistically yes.
Deb is right. Whilst technically people should not use your images without asking (you own the copyright to any photos you take), there is only so much one can do to prevent image theft. As has been mentioned, using watermarks is one approach. Dave has also designed the site to make it a little harder to download images from the database (the right-click context menu is disabled), but anyone who really wants to do so can still get around this.
In the end, it's up to you to decide whether you're happy with the risk that somebody might steal your image or not. The risk is non-zero, but generally it's not very high.