Ocheck class: Display and verify validation images

This class can be used to display and verify whether the user selected the right verification images.

It displays a form with 3 groups of radio inputs associated to 3 verification images.

The class also displays 3 random images for the user to select in the form.

The random image file names are stored in a session variable, so the class can verify whether the user select the right radio buttons associated with the picked random images.