exactly how small do you need? here's roughly 1/2 what you included.
if the size is good, you could try tweaking the contrast and brightness to get closer to your exact preference.
one simple method of getting the gradient/transition that i think you're looking for is to establish the lightest and darkest areas (drawing the centers of the areas) and then "soften" the in-betweens with the softening tool which grabs those colors and interprets/interpolates the middle colors based on the scale/size of the softening "brush".
if you're getting real small, (it's already almost there), i'd deal with it on a pixel by pixel basis and working with softening intensities to get slight gradients that i THINK you're looking for.
not sure which tools you're using, btw. i mainly use Paint Shop Pro, and some adobe/macromedia, too, including photoshop, illustrator, and fireworks for 2d stuff, but not too often.
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