The limit on image size is to keep the file storage on the site to a manageable level.. a 800x 600 image is about 1meg in size whereas most modern cameras do 12mega pixel or more which equated to about 10meg an image (or more). At the minute this forum has just short of 20,000 images (my calculator is too hot and bothered in this weather to work that out, but its a far size for a website)
Also three seemed a sensible amount of attachments to a post, if you want more say so (committee) cannot be open ended as that a classic way to bring the server down.
by all means use an offsite photo host like Flickr and link the pictures in