Oh, sorry, I think I made you misunderstand my English because I'm not good at it.
What I meant was what should I use to use an image with a weapon on the back like the screenshot below. I'm not sure what it is even though I looked at the guide and looked it up :(
As I said: holding a weapon outside of combat is not a supported animation. That means you'll need to choose and place the weapon sprites yourself. You can copy/paste the weapons into a new page 1 layer, or draw from the weapon sprite source file in-engine at runtime. But I can't help you with either of these options, I am not a gamedev instructor. If you want to do this thing, you will need to flex your brain and make it happen on your own. Good luck!