As for me, it's better use vector graphics formats, as they are to rsater graphics, which is representation of images in pixels (representation of photographic images).
Vector is always the safe way to go as most clients (not all) end up using them on other bits and bobs.. but if you know its purely for website use I think a very large JPG would be fine.
Remember photorealistic images JPG , flat cartoony images do well in .GIF