You need to bring up your Tools palette and select the brush tool at the bottom. Now right click on the boarder of the picture control and in the color palette, select the small box in the upper right with the little 'T' in it. This is the transparent color.
The attached image shows the same picture with and without a boarder.
Ed

Ed Dickens - Certified LabVIEW Architect
Lockheed Martin Space
Using the Abort button to stop your VI is like using a tree to stop your car. It works, but there may be consequences.