The great thing about using Flash for web design is that you can create complex GUIs (graphical user interfaces) rich in images without having to use tables or DHTML/layers, and without being stuck in rigid boxed-menu format and without having to do too much work involving piecing together cut-up images. While tables are good for web design, there's always a problem of cross-compatibility; just when you get your HTML looking right in Internet Explorer, a test in Netscape, Firefox, or Opera might completely throw your design apart--or, if you try them all out on a PC, you may pull up the design on a Mac and find your layout tumbled all across the screen. Using Flash guarantees that your layout will display properly no matter what browser or OS, and allow you greater layout creativity.
I've started off by creating a base layout design in Photoshop for a mock-portfolio site, using some of my own (admittedly old and outdated) artwork and some wonderful tattoo brushes from ObsidianDawn. To be kind to people with lower resolutions, I'm designing for 800x600 screens or higher; my final image size came out to be 775 pixels wide (leaving a few pixels off to take the scrollbar into account) and 612 pixels tall. My layout is designed to enclose a certain area where content will be displayed.