Assuming you use a Mac, here's what I do:

1. Open up your web app with Safari

2. Hide navigation and bookmark bars

3. CTRL+ALT+SHIFT+4

4. SPACE

5. Click on the window

You now have a sexy screenshot that will look like this: https://f.cloud.github.com/assets/98681/461852/dcd23422-b44d...

Anything in the background will be hidden, you will have a clean image with white background.