Saw your question in the ORAS mega evo thread. To do a sig, below your username on top, go to controls, edit signature, type your stuff into that box. If you want a picture be sure to upload and click the small "insert signature picture" next to the preview. Once you're satisfied (you can preview it just to be sure), click save and you're done. Hope that helps.
Oh yeah, and be sure that the picture you're using is within the guidelines. You can check them here. I use photobucket to resize my photos.