If they're going on a DV or MPEG print, 1920 x 960 is plenty. Remember that vid is equivalent to 72 dpi, so higher resolutions don't buy you much, and can potentially cause problems.
To fill the full screen in DV your pictures have to be 720x480, but not with a propotional pixel aspect ratio. Pictures from your digital camera will have a 1:1 vertical to horizontal ratio.
If you want to make them 16:9 you have to have 873x480 resolution pictures with square pixels that are later converted to 720x480 resolution with a pixel aspect ratio of 1:1.2121 (DV widescreen).
For 4:3 footage you will need a 640x480 original square pixeled picture resized to 720x480 to get he proper aspect ratio.
Again, unless you're zooming, and at that point, you'll want to double up on pixels to have a clean zoom.
I'm cutting my stills to match our Canon XL2, which is 960 x 480. Aspect derived from 960 x .909 is 873 x 480. However, I don't concern myself with setting the aspect to 1746 since Vegas scales correctly for me with the aspect scripts we use.