There's a thread for showing off your character's outfits on the last page or so, go a few pages back and you'll find it.
Also, to post pictures, you can take a picture in the game (hold down the L or R button, then hold down the other one, it'll snap a pic). Then all you have to do is get the picture from your 3DS SD card, if you have a card adapter for your computer or a card slot you can do that easily, it's the same as getting pictures out of a digital photographic camera on to your computer. If not, I think you can use a service Nintendo has that lets you upload those screenshots directly online or you could use the browser function to e-mail them to yourself or something, though I've never done that. I just remove the SD card and use it to get the pictures onto my computer.
To put photos in posts, you can just do what you did with your signature. Click the little icon that looks like a picture with a tree (next to the movie reel icon) at the top of the box where you type in your posts, then upload the image.
I hope that helps.
