Sending messages, pictures, etc. on SL75

I can send and receive messages that are just purely text on my phone just fine, but when I try to send or receive pictures, it does not work for some reason. I notice that I can send much smaller images (i.e. wallpaper images) just fine, but when I send the normal size pictures, it doesn't work. Cingular is my service provider, but they have helped me as much as they can. The SL75 is technically not one of their sponsored phones. And I cannot get thru to customer service at BenQ-Siemens. Is there something I can do to set up my phone correctly so I can take advantage of all its functions?