Maybe throw in some preferred dimensions?

IE: "While a 25,600 x 900 px medieval-tapestry-style image depicting the history of the daevite empire would be awesome, it would also be very difficult to display at the top of a hub page"

The width of the content box for pages is 707 px, so call 700 px wide a good round number for maximum width. Specifying a maximum height might be a good idea, too: Standard large horizontal web banner size is a widescreen format of 2.4:1 because lower ratios tend to take over a page, so that would be 700x300px in this case. Compare that to the ~1.4:1 ratio of a sheet of A4 paper and you've got a page where the first screen is a 700x500px image unless you crop it or reduce it even farther.

For those entering, pro tip: Take a cue from the old masters who made huge paintings meant to be viewed from across the room; working several times larger than final display size is a good idea. Just reduce the final image before posting to save everyone some bandwidth.

Also: multiple entries or no?