5
I'm using FireFox 1.0 and it is centred for me too. If yours is still to the left try clearing your cache folder (the templates_c folder in the root of your web space)
As a rule of thumb, no graphic should exceed 50Kb, even in this nice new age of Broadband, but if webmasters just up their file size then the net will seem like we're all still using dial-up lol. Let's not forget that there are still millions of dial-up subscribers too - you need to make your site as user friendly to as many people as you can. I use Adobe ImageReady mostly, and just choose 'Optimise for Web' when saving.