So throughout this site in comments and the forums, I've seen people using their Mii as their avatar. I tried google to see how they do it but it didn't come up with the answer I wanted... My question is how do I "upload" my Mii as my avatar?

you have to acsses the internet through your 3ds or wii and when you edit you profile the picture of your mii should be there. If using 3ds make take a head or body shot of mii before you can have it in your photos

What about the Wii U? It says I can save my Mii as an image but I need a SD card... Is that correct?

I do believe this site provides exactly the feature you desire. Obviously access it from your 3ds, and it offers step by step instructions on how to use the site. In my experience however, it takes a while to load, so be patient. :3

I don't have a 3DS or a Wii, only a Wii U... =/

I'm yet to use Mii Maker on Wii U outside of setting up my personal Mii, but that should work. I used to have my Mii as my avatar and I imported it by saving it to the SD card. Resizing your image to 80px by 80px with something like PicResize also helps it look nicer as an avatar(when really large pictures get auto-downsized, they can look really fuzzy).

I did it through my nintendo network premium page, somehow it had my avatar there so I saved it to PC, then made it my avatar like that.

Thanks for the information. Finally got my Mii to be my avatar [: