If you want to try to figure out why the picture is called "Prime Yolk" stop reading, because I'm about to explain. First, and most obvious, what's pictured is the yolk of a chicken egg. The rest is math and composition. The first eight prime numbers are encoded by the composition, in order as follows: the diameter of the yolk, the distance from the top margin, the distance from the left margin, the distance from the bottom margin, the distance from the right margin, the distance to the lower, right corner, and two more lines. The final two lines are approximated by the lines in USPS, elsewhere in this gallery: take the point of the intersection of the lower, horizontal tangent to the yolk and the left margin. The distance from this to the bottom right corner is the seventh prime. The final prime is the diagonal of the frame. The final three primes, of course, are just approximations.