you need to look at what bandwidth your viewers are likely to have.

a 280 kb picture will take about 40 seconds on a 56k dialup and 8 seconds on a 256k dsl line.

The  other issue is how much data transfer does your host allow each month ?  you can do a rough calculation to see how many page views that would be.

With jpeg you might get better compression for negligable image loss although this is a matter of opinion on whats acceptable.

A page of your web comic angie1.jpg saved for web from photoshop changed from 280k to 96k and to me looked fine.