Hey Mark, at one time I thought I wanted to be a web site guy; until I dealt with my first client! What a nightmare! But that's a different story for another day.
When I do my web sites I design them to fit in a browser displayed on a screen running at 800 x 600. To do this, I set everything inside a main table sized at 750 x 475. That leaves enough space on the sides to allow for the default scrollbar gap on the right of the screen and enough space top and bottom for the default configurations of buttons bars and status bars.
You have to keep in mind that while there are a lot of people running at 1024 x 768 or higher, there is at least an equal amount of people running at 800 x 600 or lower, either due to old computers or bad eyes!
My newest Vortex Imaging
site (whose layout was actually designed by a different company) is an exception to my rule and is setup for a 1024 x 786 screen although (to my surprise) it scales down to 800 x 600 fairly well.
Now, that being said, I hope you're not really planning on using a huge picture on your first page? Even with highspeed connections I wouldn't make your picture any bigger than 30K in size if you can at all help it! I make my viewable shots a fair size so people can see them decently. But I try to make the pages themselves load fairly quickly so people don't have to wait to get around the site.
I hope that helps. Oh one other thing. I get a lot of complaints from people about NOT having a BACK button in the page design. It seems that people forget there is a back button on the browser that works just fine! But you might want to think about including such a thing anyways.