In terms of arguments:
Can you tell them that having a site that can't be seen on blackberries (and cell phones in general), that can't be seen on low bandwidth connections (and that include a lot of wireless connections on laptops) is really unprofessional. Putting stuff into flash that does not need to be put into flash is a widely made mistake, comitted b people who don't know that they are doing. Would it be too harsh to say that you think too much of this company to have your site make mistakes that label as amatateur hour?
Point out that google does not use flash or high res graphics.
Also point out that in general, super high res graphics don't belong on either the main page or integrated into into pages with a lot of text information. The professional way to use high res graphics is integate low res graphics into standard pages that serve as links to high res.
Are there exceptions to this? Sure major ones: sites whose business model centers around media - Disney, Tv networks that sort of things. (Even then I think it would be bette if the fuckers did not put flash video on the main page.)
I will add that a flash site not only excludes phone users and many other mobile users. Most journals ban flash from the newsroom, because of the security risks. So if you make your site flash-centric you are also making in inaccessible to the many journalists. Is free publicity unimportant to your business?