Default change messaging wallpaper

gud day xda. new g2 user here. im not sure if this has been asked before, but how do u change the wallpaper on the messaging app? i dont like the wallpapers that came with the messaging app and everytime i try changing the wallpaper with a photo from my gallery, it always says that there isnt enough memory. by the way, im using a rooted d802 running stock rom. thanks....