Thanks, Lugia.
Your advice has helped me improve the image. In addition to reducing the dpi, I cut the overall dimensions of the image 75%.
It is still blurry, but it is similar to other module images and may be close to the best the site can provide. Ideally, I would like all labels to appear clearly, but the necessary bandwidth might require a lot more unicorns.
Would be nice if there was simple guide about the number of pixels per hp/U for uploading.