St Matthews 20 sec by 2010kev, on Flickr
Ugh, flickr forced me to change to the new layout, how vile.
When you click the Share icon, look just above the text box and you'll see a photo size drop-down. Medium 800x530 is a good size. You'll need to click the pushpin icon to select forum-style tags before you set the size or it will just revert to 500 pixels wide.