You are posting pictures using the stangnet.com server which has limited space. You must go to another website that holds your pictures (photobucket.com, imageshack.com, cardomain.com) and when you want to post them on this forum, you link them from your host site.
I think the lower the resolution the pictures you produce, the greater number of pictures you can post. Just like on you memory card for your digital camera; the better the quality of the photos, the less pictures you can store.