Picasa has a feature that will allow you to fix an image that is slightly crooked. First select the picture and double-click on it in the Lightbox to open the Editing Window.
Select the Basic Fixes tab and click on the Straighten button.
In the Straighten Picture view, use the slider at the bottom of the screen to align the picture against the boxed grid. Click Apply to save your changes in Picasa, or Cancel to return to the Basic Fixes tab.