screen capture

[Lowell Neudeck]

When using using cmd-shift-4, I can take a picture of a portion of the 
desktop or part of a web page. When I double clicked on the resulting 
image, Picture Viewer opened the picture. But now, the resulting 
picture appears on my desktop as an Adobe PDF. When I double click on 
it, Adobe Reader, instead of Picture Viewer, opens and the selected 
picture appears. How do I change it so that Picture Viewer opens?

TIA
Lowell