TL;DR: UPLOAD IT TO IMGUR.
Honestly, all you do is drag the image onto the page, and it'll say "UPLOAD" in the background. Just drop the image down and click "upload" (You can tick the little edit box if you want to resize your picture or edit it in some way). It'll take you to a page with your image and a bunch of links on the right-hand side. You want to copy the "Direct Link". Any time you paste the Direct Link on wikia, the image will automatically embed without being added to our gallery.