Same a for video. When you've added the still to your timeline, zoom and pan with the pan/crop tool. Use keyframes (read up on keyframes in the manual).
Tor
This is a great feature. From media pool or explorer find the picture and then add it to your timeline. If you'll notice, there are a few tool buttons on the picture on your timeline. One of them looks like a square with a couple of the corners overlapping. Click that for panning and cropping. That should bring up a window that has your picture in it and a bunch of cropping tools.
The key to all this is the little diamonds on the bottom. Crop your picture to where you want the clip to start panning. Now the rest of the line to its right is time. Click somewhere on that line (probably the end if you want one smooth pan) and then click on the diamond with the plus sign on it. By doing this you are adding a point in time. Now crop the picture again where you want the pan to end and voila!
You can add as many points as you want to change direction etc. These diamonds are used in other effects also, just remember they always represent points in time.
Vegas' capabilities to manipulate stills is one of its unsung features. In addition to panning and cropping, the ability to make changes to brightness, contrast, color correction and see the results on an NTSC monitor is an enormous time saver when doing large slide shows. (The alternative being re-loading the image after alteration in Photoshop/PSP, etc.)