thumbnail is the upload way here for the new forum. I would guess that if you wanted to "post them big" you'd post it from an outside link and use html i.e. [url]www.blah.blah.com[/url] and [ /img] or whatever code it uses here.
thumbnail makes it nice when folks post pictures that are so huge they screw up the screen where you ahve to go side to side to look at it (i just won't most of the time). Disadvantage of thumbnails is that the picture is small and to look at it bigger you have to click on it to see it in a new screen.