I believe you have two options if you already have a website. 1. Replace the website with a flash or combination site including flash 2. Add a flash file within your existing site. I will assume that you just want to update the existing site and want more movement (interest). The comments made previous to this are all relevant on the design, but assuming that you still want to add the Flash then here goes. Flash files that have great imagery can be purchased from many place search for Flash templates I am not the super techie, but to embed them into a site you need to have a .swf extension rather than the .fla file extension from the source file. Hope that helps.
Both WordPress and Joomla are excellent, widely used, well supported CMS options. I use Joomla because it is so widely used, always getting better and is very intuitive. There are excellent templates available and support and expertise is easy to find.