I can't figure out how something so simple was done..

http://www.Planet-Source-Code.com/vb...50597&lngWId=1

I was interested in this code and noticed something very simple that I notice in alot of VB projects from PLC... the background of the images are already transparent/\.. or just the same color as the form.. I couldn't figure out how this was done... Can someone please explain it to me?