Open Pan/Crop for the event and click on Match Source Aspect. When you want an oddly shaped image to fill the screen you click Match Output Aspect.
Tor
Oops - seems it didn't work quite like that. You must match your preferred aspect plus resize, either in Pan/Crop or Track Motion.
Unfortunately there's no way to control what vegas does with a still when you drop it on the timeline (although you can set up a preference to import a specific file type as 0.9091 PAR)
It's dumb and a failing of Vegas.
What is needed is yet another preference setting to set how vegas handles stills dropped on the timeline:
Fit in frame
Fill Frame
Actual pixels
Always ask.
Also, Vegas should always ask what you want to do if you drop a slew of images of varying sizes.