I notice that there was a blank horsey thingy on my profile. So how do I change it?

Do you see the MY ACOUNT tab, near the top of the screen? Hover your mouse over it and click the My Account, scroll down to Prefereneces, and you'll notice that same picture. To the left of it, you'll ntoice a Choose File button, that's what you need to click.

Less wordy:

Go to My Acount -> Scroll down to Preferneces -> Click Choose File -> choose an image.

I'm not sure if it can only be a set size, like Custom Skins, since I haven't changed mine, though.

The picture will be automatically resized for you when you enter it as your profile picture, so no need to worry about that.

Also after you click save prefrences refresh the page. This will set the new picture.