The limitation is there within SMF for a reason. Ive already increased it from the default and have many, many times increased storage capacity in the attachment folder. It is surprising how it soon fills up and I will be frank in saying that I dont have any intention of reviewing this. Its not as important now that I have my own server, but there were times in the past when I had to manually go through and delete large images because they were taking up too much space on the server.
Storage limitations aside, some people post mega large images that are too big for the width of a forum, so that is why the SMF limit is there.
However I think I understand what you are trying to do. There are a couple of alternatives
1) Host the images elsewhere and use the [img] tags to display them.
or do what I do:
2) Make a post add 3 images, then make another post saying 'next set of images' and attatch those, etc etc until your done.
