Align text field over an image - Help!

I need to align a text field over a screenshot of a textfield so it looks to the user like they are actually typing in the text box on the image.

This is easy in a blank project, but I need to be able to do this and it fit on different monitors.

I have tried multiple approaches, including: Fluid Boxes, positioning techniques...

I dont understand why this is so incredibly difficult on software like captivate.

This is simple CSS.

Can anyone help me figure out what I am doing wrong?