I'm not sure how helpful I'll be, since I have the same problem when doing avatars, but I’ve noticed that the closer the canvas size is to the size to need to make it, the less blurred it gets. There will always be a small change when you either resize an image or convert it to a PNG, JPG, or something similar.

So, there are really two options: make the canvas as close to the size you need the image as possible or make the canvas huge and make everything sharper than you would normally. This means minimal blurring/blending in the image, since resizing it will likely do most of that for you when it reduces the quality along with the image. Small details like the scars are going to blur at least a little bit no matter what you do to it online, so resizing by using your bio or something set up like the forum text boxes that let you resize images might be a good option too, since there was hardly any blurring in your image when you did it that way.

Hopefully this helps at least a little bit. Please let me know if you need me to explain it better. It looks great, by the way!