A while ago I held a webcast and installed the optincrusher live for a few websites, including customizations to make it fit with the colors and logos.
More than just teaching you how to customize the optincrusher it turned out to be a fantastic tutorial about using a free graphics program (Inkscape) to create custom images for your website. I actually show you how to use Photoshop a little too.
You can download the HD version below the video. If you watch it here on this site, click the full screen button so that you get all the details.