For specific pages--- In edit mode--- under add features and media--- Photo -- there you can find or upload the image (you can adjust their size too)
also there should be a contribute box where you can "add a photo"
should you wish to upload it to wiki--the red bar at the top--- under "on the wiki" or "community"--- look for "new photos" and there you can upload new photos
More reliable info: Here
Remember follow the wiki rules...which means no fan art eg. colourings, gifs, etc :) (don't know about fan-made translations though...ask admins on those) and don't vandalize pages :D