Imgur is pronounced “image-er/im-ij-er.” The name comes from “ur” and the extension “img” – your image!
But it’s not an intuitive pronunciation. When I first encountered the site I called it “im-gur” or “im-grr”. Because the g is followed by a u, it didn’t even occur to me that it might be a soft /dʒ/ sound. Most of the people I’ve spoken to about it agree, or they avoid saying it altogether.