Custom Login Page - Size Issue

Custom Login Page - Size Issue

Hi

On my custom login page I've noticed sometime the background image is resized for some of my technicians.  

This is what I want it to look like:




But sometimes it looks like this:




This is the code:


<div class="sdp-bg" style="background: url(/custom/customimages/FINALPTLOGORESIZENEW.jpg)"></div>

Is there some additional code I can add so that the login resizes for all my techs regardless of their resolution?