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

Hello! I am currently working on some personal css skins but I was wondering if there was a way to replace the Neit icon with something else? I looked around for a bit but haven't found anything yet... Was hoping I may have a better chance here. Thank you :] EDIT: I also wanted to ask to be on the safe side, does that icon have to be a specific pixel size or does it just have to be a square?