As noted in that Wikipedia page, there is also the MNG format (http://en.wikipedia.org/wiki/Multiple-image_Network_Graphics http://en.wikipedia.org/wiki/Multipl

As noted in that Wikipedia page, there is also the MNG format (http://en.wikipedia.org/wiki/Multiple-image_Network_Graphics http://en.wikipedia.org/wiki/Multiple-image_Network_Graphics). Unfortunately it's over 10 years old and there is still little browser support.