When you upload a photo to Facebook it automatically resizes the dimensions and resolution to fit Facebook's own requirements, that way they can control how much space is used and how quickly images get displayed.
If you want to post a picture on this or any other forum (e.g. ATF) you have to use a hosting site. Although the pictures are displayed within a post here, they aren't stored here, they get pulled in as people read pages.
Theoretically we could host photos on this forum but the free service we currently enjoy would be filled in a few days. We could probably buy additional storage space here but we would have to charge an annual membership fee and probably limit image size, numbers, etc.