Need help making background dynamic

need help making background dynamic, i uploaded pics to pexel and splahbase but cant find them at all. Not even from the auctual sites even though i see they’re uploaded from my profile. Is there another way because the static image messes up when i PREVIEW my program.

Can you open your app so that other users look at it?

yes i am still building it and i can preview it but the picture is static from my files. I need it to be dynamic so it can respond to the changes in size.