Yes a lot of these images, have high enough resolution that you can use them for printing billboards. A good way to determine the appropiate resolution is to figure what % of the page they will cover.
So if you have a site that's about 960px wide and you want each photo to be wide enough to cover 2/3 of that then you must reduce all your images to be 640px.
as said before this can easily be done in any RASTER graphics program , such as Gimp or Photoshop, even ...gasp.. Paint.
however , Photoshop does have a neat trick. You can RECORD an ACTION ( look for the action palette), while reducing and toning each image to your desired size, EXPORT that action as a DROPLET. Then you can just DRAG and DROP ALL your IMAGES to the droplet and they wil be resized to that exact size as the first one. This saves both time and keeps your work consistent!
WARNING , make sure to save a back up of the original images.
Hope that helps.