So… to use them I have to upload them to my uploads right?? or is there a different way?
Yes. But you still need to resize them.
You can do that here.
One panel is 640x1136
Two Panel is 1280x1136
Three Panel is 1920x1136
One panel isn’t a good idea since the picture will be smaller and it looks strange. Just resize them to the panel amount you want and then Upload them on Backgrounds.