I would imaging Jpegs/Png would be best if your working with a set size(s), but if your looking at objects which resize them selves you would best with vector images. Most games don't use vector graphics because the objects they are being used with don't have their sizes resized that much if at all, of cause if your making a game such as spore... you might not even want to use graphics at all and just change the images using hex code.