disable print screen

function?

Yes. Just check the 'Disable Print Screen & Clipboard' box in the main HTML Guardian window before you encrypt html file for which you want to use this security option. This will make impossible taking screenshots of the protected html file.

Note that using the disable PrintSreen security option is not enough to protect images (or whatever other content) on your site. It is just an additional measure. Web site thieves rarely use the PrintScreen feature because any image taken this way is with a much lower quality than the original image file.
The quality of an image taken through the print screen button depends on so many factors like the visitor's screen resolution, type of the video adapter, colour depth and a lot of other operating system settings or hardware related issues. Futhermore hitting the print screen button produces a bitmap (.bmp) file. Usually to use it in a web page or for most other purposes, you will have to convert this file into a more popular and widely supported format such as .jpg or .gif. Of cource any data convertion from one format to another (such as bmp --> jpg or bmp --> gif) reduces the quality as well.

The most common way to steal images is to right click on some picture and then select 'Save Image As' from the context menu. If this is not possible, most thieves will look into the source code to see where the image is. Of course, HTML Guardian will hide source code so such information can't be derived from it. Another widely used approach is to use the so called image toolbar which is available in IE browsers (version 6.0 and later).
Disable print screen can't protect you against them, but you may use the Disable right click feature.
However when somebody browses your website, a copy of each file referenced in it remains in the cache and can be later retrieved from it. Neither disable print screen nor disable right click can prevent this. Only using the ImageGuardian ensures complete images protection.