Conversion alone only brings it down from 3.6 MB to about 700 kilobytes. I tried to then just resize it to 52% to make it exactly 300 kilobytes, but wikia will not stretch it to normal size. That means only about 1/4 of the top left would be covered, or it would turn into a repeating pattern. Then if I try to resize it myself to make it big enough again, it'll become well over 300 kB. The only way I could do it is by convert to jpeg, then resize to 30%, then resize again to 300%. The result is extremely pixelated, with large pixels.
Kronos has some special program that he used to create the JC wiki background.
Or I guess there's one more thing I could try: I could resize it to something and then add a large white blank space in the middle. Previously my plan was to make the wiki content area a bit transparent (like now), so the middle of the picture would be visible too.
471 x 115 pixels. Done. Wikia automatically fades the left edge of it. I had to cut a little away from the top and bottom of the picture, because that artwork is a bit taller when resized to 471 pixels wide.