View the frame you want in the preview window, set the preview quality to Best/Full, the click the Save icon at the top-center of the preview window. This will open the dialog box asking you where you want to save the image and you can select either jpg or png.
Note: Be sure your project settings are at the size of the image you want (1920/1080 is the default). Also if the video is interlaced, you'll likely want to de-interlace the image in a program like Photoshop.
I was using the "Open in Graphics Editor" script last week and it was failing. I realized that is was because I was in Vegas 64bit. It only works in the 32bit version.