When you go to the wiki, on the left side bar, click the link where it says "upload a new image". Select the pic you want and upload it. Looking at the Recent Changes, I'm not seeing where you've uploaded the pic yet, so that's step 1.
Then go back to your character's wiki page, go into Edit, and paste in the file name for the image where it should be in the info box.
Then go back to your character's wiki page, go into Edit, and paste in the file name for the image where it should be in the info box.