There are basically 2 ways to make thumbnails. You can save your full size image in your fractal generator and then change the size to 200x200, but that doesn't always get you under the file size of 15k. The other way is to take your full size image to a graphics program like PhotoShop or Paint Shop Pro and resize it there and saving it at the quality that will give you a file size of 15k. If you don't have those programs, you might want to google 'Gimp' or 'Irfanview'. These are both free graphics programs that many of the artists here use for postwork of all kinds. HTH

Fractals will always amaze me!