Image formats with transparency. Best options filesize-wise?

In the game i am working on I have just finished the main character who has a good number of images that need to be loaded for all the various actions. I’ve only gotten half way through joining them all in one file and the png with transparency with all the frames is already up to 500kb. This seems like it could get out of hand pretty fast, especially when animating other characters, backgrounds etc.

Assuming i need my graphics to be bitmap based, and more often than not with transparencies on them, is there a more bandwidth efficient way of doing this than pngs?