In case you are talking about the images for the related pages from categories (Read more), you can simply add a link to this image- wikia converts them directly to images from links. You can right click one of those images to find its link.
The categories images are veryt similar to ordinary thumb images, but they contain anotehr part in the ending. It contains 4 numbers, and the overall ending would look like this: 200px-10,1200,120,720-Aqua2.PNG
red-> the width (same as in any other thumb image)