I've actually developed an entire theme so I'm familiar with the tumblr theme code. if you want to I can help you with this.
I'm a little puzzled though, isn't newest first what you want? as in: when they visit your page, they see the latest panel, and those who want to go back in history can use the nav buttons. (might be nice to put in regular first/previous/next/latest buttons.) that's how most webcomics do it I believe.
hmm....well..you've got a good point there, i never thought of it that way
....HMMMMM... i was thinking, that new people would automatically see the oldest post first and then you'd start out, but i suppose you ARE right, a lot of webcomics do it the opposite way and if youre new you just redirect yourself to the start........