A crude method, check its user-agent and block that in your script, the second, better method
is to use turing codes when a user posts or more conveniently logs in, so that a script, without
a human viewing the image cant post to your site. Lots of sites are doing this now
such as hotmail have added it to their login.
Here is a good place to start
There are some good snippets to get you on your way, if you use php that is.