On the character's page hit the edit button along the tab at the top. At the top of the editing page you will see a section that looks something like:
Anywhere in this section you want to insert the following:
|image = name.png
|imagewidth = 200
|caption = Image by WHOEVER
Replace the name of the artist with the actual name and the name of the image with the name of the character. You do not have to link to a valid image here, this is the location you are about to upload to. After you've done this hit preview and you'll see something like:
Click and open it in a new tab. On that tab select your picture and click upload file at the bottom of the page. After this has uploaded go back to your first tab, and hit preview again. If everything is correct the infobox will now contain your picture. Should the picture look distorted, or too small adjust the number after imagewidth and preview again. Do this until it looks okay.
Finally:
Make sure to remove this category to remove the character from the list. Once you have done that hit submit and you're done.