There are a few things to know that will help you as you view pictures in Picasa.

Thumbnail Size

To change the size of your thumbnails in the Picture Library, click and drag the Thumbnail Size slider at the bottom right of your screen.

Keyboard shortcut: hit CTRL+1 for small thumbnails, or CTRL+2 for large thumbnails.

Thumbnail Caption

To display relevant information about your pictures under the thumbnails, open the View menu and choose Thumbnail Caption. You can choose to display the filename, an excerpt of the picture caption, or the image resolution.  

Pan / Zoom

To zoom into a picture, drag the zoom slider to the right, or hold the up arrow key. When you are zoomed in, the Zoom window will appear. To pan around inside the picture, click and drag inside the Zoom window.

To view a picture at its actual resolution onscreen, click the 1:1 button. To zoom out, click the Fit button or hit the Escape key. To view a picture fullscreen, highlight it and hold down the CTRL+ALT keys.

Rotate

To rotate a single picture, click on the picture you want to rotate, and click the Rotate arrow. To rotate multiple pictures, shift+click or control+click to select multiple pictures, and click the Rotate arrow. If your digital camera supports auto rotate, Picasa will automatically orient your pictures correctly.  

You can select a picture and use ctrl+R to rotate clockwise or ctrl+shift+R to rotate counterclockwise.

Locate on Disk

To find an original source file, right-click a picture and choose Locate On Disk. An Explorer window will open and display your file on disk.