How to Set Picasa as My Default Photo Viewer
- 1). Open Picasa and then click "Tools" and select "Configure Photo Viewer."
- 2). Select the "File Types" tab and then click the box next to each image file type you want Picasa to open. Click "Select All" to use Picasa for all the listed image file types.
- 3). Click "OK" to make Picasa the default viewer.
Source...