I'd make sure there's an agreement in place before you hand anything over. Remember that you can't actually prevent them from doing anything, but you can lay out what happens if they do.
Your images are protected by copyright, but your agreement should spell out the rest of what's expected. In addition to the standard "What's delivered/how and when you pay", you can lay out the following:
- Whether you get the source they develop (as opposed to just a publishable copy of the site)
- Whether they can take what they've developed for you and use it for somebody else (replace your logo and then sell some other customer the site
- What happens with any other custom components
- If they're allowed to talk to others about your work (NDA)
Before you hand anything over, at least have a basic agreement in place - if you haven't paid and you cut ties, you can at least require them to destroy what they've received from you, rather than fight with them over what happens when the relationship ends.