B4J Question Draw X on picture


I am creating an application that makes it possible to fill a form which is a PDF turned into an image file.
After filling some textfields and checkboxes these should draw X in the boxes on the image.
How can I draw two lines on the image to represent a cross (X) in the form boxes ?
Later I want to make this possible on Android and iOS since users should be able to fill the form on a computer
as well as in their phones. The idea is to then email the picture as a complete filled form.