Reg:Image Capturing on logon page

Hi All

Is it possible to add capture image box on saplogon page ,when end user login with portal user id and password before he click on login button

he need to enter the capture image text in the specified box and needed to sign up.

These things we have seen in some sites where it provides some automatic text when we enter the right text in the given box only it vl make us to allow in.

But How can we meet the same approach in sap portal logon page ,what are the steps do we need to meet the above requirement .

Can you guys please suggest me wether this scenario is possible or not ?

If not why ?

If its possible what all the prerequistes do we required for it ..

Thanks & Regards

Deepika