Single post in Site Skins: How-To, and Helpful CSS

The main trouble with the trainer image replacement is: I'm not sure of a good way to detect whose profile you're on with CSS alone. You could do it with a userscript, or you could use a CSS extension like Stylus, and set it to only be enabled on your profile page. Either way wouldn't work well if you use multiple devices, since it wouldn't be in your skin... but if you're interested, I could explain a little more - the Stylus method would be the simplest.