avatar, banner and background
the three images on your page, and what each one is for.
three different images, set in two different places. it is worth knowing which is which before you upload.
uploading
avatars and banners open a cropper so you can frame them. the banner crops to the shape it actually renders at, so what you see in the cropper is what lands on the page.
if you have discord connected you can skip the avatar upload entirely and sync your discord picture instead, so changing it there changes it here. your discord avatar decoration can be shown too. both live in integrations.
backgrounds are their own thing
a background can be a color, a gradient, an image or a video, and it has its own overlay, blur and position controls. backgrounds covers all of it.
still stuck? ask in the discord, someone will know.