A very general rule I learned is that png's are best for images with large areas of the same color (screenshots, logos, text) while jpegs are best for images with lots of color variation (real-life pictures, for example).
Is it wrong that I've been to these sort of conferences and actually enjoyed myself?
Webstock (http://www.webstock.org.nz/) in particular is great if you're ever in my part of the world. Great signal-to-noise ratio, fantastic speakers, very little of the standard tech conference douche baggery ("10 tips to monetize your blog"; any panel comprised of commentators rather than people who make stuff; etc.) and an upbeat sort of atmosphere that's difficult to articulate.
In my mind, the key differentiator between mediocre events and great events is who the target audience for the event is: