Test your .htaccess mod_rewrite hotlink code.
Enter your image URL and click "do it" - if you see it, you need to try again!
While most browsers will grab a fresh version of the image when you simply refresh the page, some won't. If it's an image you have recently viewed, don't forget to clear your browser cache/restart your browser before testing, or you could easily end up screwing completely workable hot-link code without realizing it.
A good system, I find, is to have a directory listing open in one browser tab; a folder of images on the server in question, somewhere covered by your .htaccess rules, preferably filled with new images, images that have never been viewed in a browser, and for each hot-link test, simply use the next image. In other words, don't use the same image twice.
If you are looking for a way to get instant-payback from sites stealing your images, checkout this.