I've actually explained this to a client of mine the other day. (The site he paid someone to design was literally 99% images with barely any text or SEO attachments.) In the end I ranked the client's site for a bunch of competitive keywords, but it did take a LOT of extra off-site work on my part and I did SEO optimize the images. (it also took much longer)
Sure, titles, descriptions, alt text, etc have some value... But IMHO Google can't "see" a picture to give it value. An untitled.jpg clown picture is going to rank the same as an identically sized .untitled.jpg of the "Mona Lisa" and so on. This is obviously excluding manual reviews, but that's usually after you are ranked and in that case it's usually just a "checkup".
There really shouldn't be some magical number where images start hurting your site. In my mind it'll start to hurt you when the images or even things like flash start replacing your textual content or when your load time starts increasing. I always make sure the sites I work on have at least some meat for the Google spiders to chew on as well. It helps me more than anything in the long run... ...So if i can pull some clever SEO safe coding instead of kicking up Photoshop, i will.
Just my 2c