Picasa uses the standard Windows mechanism to open default programs (applications) for viewing photos and videos. Follow these steps to add your favorite default program to the "Open with" menu in Picasa:
In Picasa, right-click a photograph or video and choose "Locate on disk" - Windows Explorer will open
Right-click the image in Windows Explorer then mouse over "Open with"
In the menu that opens, go to the bottom and select "Choose default program"
Select the application of your choice under "Recommended Programs"
If your program of choice is not listed, look in "Other Programs" or click Browse to find the program on your computer
In Picasa, when you right-click a photo, the default program should appear in the "Open With" menu