If you are creating a dvd for normal television the image size is 720 x 480 x 16bit color The file on DVD is an mpeg 2 file format so your combined image files will be effectively rerendered through the appropriate codec. The dpi is for printing use, does not affect the render time at all (you are rendering a number of pixels x color depth). Changing or setting the DPI will change the actual print size in most paint programs. If you want some suggestions try newsgroup rec.video.production at Google groups.