android - How to create a background variable in App Inventor, so i can change the background image of a canvas using spinner -

i'm working on simple app using app inventor, visual block coding. have canvas , able change canvas background image spinner.

i have got spinner can't blocks make spinner work. on spinner (background1, background2) , below background changes depending on option choose spinner. tried looking can't find anything.

set canvas.backgroundimage in spinner.afterselecting event