all you need to do is make sure it results in a 655x480 image, assuming you want the scan to fill the screen. You don't need much resolution at all, really.
Before Photoshop CS and its video presets, I would usually prepare a 720x480 image and then resample it to 655x480 to properly handle the pixel aspect ratio, but this is really only an issue if you really need to make sure that circles appear as true circles on video. For most photos, the distortion won't really be noticeable.
Actually there is no need to resample; there won't be any distortion. Scan it, then put it on the timeline. That really is all you have to worry about. Circles will still be circles without doing anything to them at all.
If you want to make sure that you scan an image at least 654x480 and not enormously huge either then these are good things to consider as well.
If you are going to zoom or pan on your pictures for the Ken Burns effect, you may want to have a higher resolution. For a basic pan/crop, you may want to use 2x normal resolution. For something very intricate, like you zoom in on a map and the map moves from NY to LA, then you want an even higher resolution, because you would be zoomed way in.
If you are mixing stills with DV you don't want too high a resolution because they the DV video looks bad.
Set to 150, turn off any color correction, levels adjustments or scaling in any scanning software you use unless your scanner is a higher end model. If you're new to scanning, the following site has a very nice tutorial collection on how to get the most out of your scans: