I don't have a link to a good PNG heightmap, but I used Wilbur to read the ETOPO2 data and write out an equal-area 8-bit heightmap based on that data set. The histograms for each area shown below. The hump to the left is the oceanic abyssal plain and the big spike is the coastal plain/continental shelf. A straight histogram on the Plate Caree projection of the original ETOPO2 data set has some artifacts as shown in the second one (that hump off to the right in the plate caree projection is due to polar distortion for Antarctica, most likely).

