By default, you can find a gray image as a cover. To change it, click in the top right corner of the image. You should see a Camera icon appear, to be selected.
In the new window, you can choose to Upload a new image, then find an image you'd like to replace it with. To finish it, select "Update Topic". The new image, if it's of the correct dimensions, will be set as the Topic Cover. If it's not of the correct dimensions, you will get a tool that will allow you to crop it to the required dimensions.
The recommended size for the Topic cover is 640 x 640 pixels.