WordPress.org

Can people still steal my pictures?

Yes - there is no practical way to prevent someone from stealing your images. All this does is make it a little more confusing for non-technical users.

Why bother, then?

Actually, sometimes making things a little more difficult is sufficient. Ever heard the phrase, "Laws exist to keep honest people honest."? Enough said.

What is the cost?

There is no cost for the plug-in (obviously), but it does process each post as it is being loaded to display, so there is increased server load. This is negligable for a single post, but on a busy server you might want to do some benchmarking if you are considering this type of thing.