[Lynne]

From the help popup on the Image Verification page:

Image Verification Library
In order to generate verification images, vBulletin must have access to an image processing library. There are currently two libraries with which vBulletin can work: GD and ImageMagick.

Of these, ImageMagick is generally preferred, but in order to use it you must ensure that it is installed on your server.

If you can see both the GD and the ImageMagick versions of the image, you can choose either library.

If you are unable to see the ImageMagick image, it either means that you do not have ImageMagick available on your server, or that you have not entered a valid path to the ImageMagick binaries in your Image Settings.

and from right under Image Verification Library:

Choose the image processing library that vBulletin will use for generating Image Verification. If the GD (True Type) option doesn't work, please ask your host to recompile PHP with FreeType 2 support.