I won't reprint the entire tutorial but the topics below will show you what menthods Michael Irwin will teach you to implement in his Protecting Images Using PHP and htaccess article.

****
Image protection on the internet can be quite tough. With view source, and tools like Firebug, it takes a combination of tactics. In this article, we'll go over a few techniques that can be used.

Sources to check

In this article, we're going to tackle a few different tactics to safeguard images. But, let's first think about what ways people can get images.

* Direct path - if a user knows the path to an image, they can simply go straight to it, and download the image.
* Drag and drop - in most browsers today, you can simply drag and drop an image right onto your desktop.
* Hotlinking - another website can link directly to your image
* Screenshot - a visitor can simply take a screenshot of your page and crop the image.

How to fix them

So, now that we've established where our problems are, how can we fix them?

* Direct path - we're going to create a PHP script to "hide" the direct path.
* Drag and drop - we're going to implement something that Flickr does... placing a 1x1 transparent GIF on top of the image.
* Hotlinking - using .htaccess, we can prevent hotlinking from occurring
* Screenshot - not much you can do here. The best recommendation I have is to add a watermark to your image (not covered in this tutorial)

Read the rest of the Tutorial
*****

I thought that transparent gif method was a great tip!