Don't use flash.
Don't exclude people who don't have flash, a particular media player, any special plug-in, or who don't accept cookies. Make it possible to use the site in a rewarding way for people who don't want to load up on flashing-neon-light software.
Do use CSS. It allows you to edit the content and layout independently, and often displays better across browsers.
Try using .png image format for your images. It's lossless, compressed, and contains gamma information so that it displays more consistently across different platforms if your browser is a decent one.
Review the results in a number of browsers.
Check it for W3C compliance.