[callmeed]

JPG for photos; PNG for logos, graphics, ui elements, sprites or anything else that has a vector (adobe illustrator) master.

I'll probably get some downvotes for this, but IMO hacker founders shouldn't get their panties in too much of a bunch over this (with the possible exception of mobile stuff). Work on your startup and solving a problem–then– when your traffic warrants it–you can start obsessing over a few kilobytes of savings. Swapping out graphics in your page templates or CSS is trivial.

When I see a good-looking site, I don't need to go digging into their source to see if they used JPGs or PNGs before it catches my eye.

Follow the above rule, keep your master PSD/AI files, and you'll be fine.