To start you need a hosted image that is 200x300 in dimensions. If you don't have image editing software, a great site for cropping is here, you can set the aspect ratio to 200x300 and then capture the dimensions in the image, then resize it to the dimensions after. You then need to hose the image on a site like imgur, imgbb, nickpic so that you can get a direct link to it. A direct link has the file extension at the end, and looks something like this https://i.ibb.co/4KjmYFV/f5f.jpg.
Lastly scroll until you see the field for Hover Image. Paste your direct link there and save it. Currently the mini profile is the only place it is visible, so you will see it when you go to post. At a later time the hover will show up in the member profile when I get around to coding it in.