The only way to post large photos is to use a file hosting site like Photobucket, Imgur, etc.
Photobucket makes it pretty easy to do. All you do is login, upload your photo, click the gear icon on the photo, click "get links", click the field under "IMG codes", then come back here and paste the code into your post.
It will show your pic in it's full size. For example: the picture of the blue snake above is 568x640px, I usually scale my pics down before posting so that they're not humungous in the forum.