Ok so you need to go to 'Account' > 'Settings', there you'll find 'custom avatar' and 'profile banner'. You need to host your images on another website (like imgur) then put the links to the images in the the boxes for 'custom avatar' and 'profile banner'.

While banner says size of 1060 x 200 px, it doesn't say anything but I tried things and turns out the maximum size for avatars is 150 x 150.

Test

You will also need to make sure your imgur link is a direct URL. It should have the file extension listed at the end (like .png or .jpg) 

Usually when you get the imgur link, i've found that you need to right click on the image and click 'copy image address' 

So for example the correct link to use in your forum signature would be https://i.imgur.com/PZ3RIDE.png

How do I even like, mak a banner

@Kaylie Eaves, you can use software like Canva, GIMP, or Photoshop to make it, then use a site like Imgur to host the photo.