Startup Wallpaper Photo

About a year or more ago, when I would turn on my MacBook Pro, one same photo out of my file of many wallpaper photos would show at startup every time. I didn't "tell" it to do that, but the same photo was always there when I turned on my Mac. After it finished booting up, the Shuffle that I had set up would start and continue. But when I shut down the Mac and started it up again, that same photo would be there again. Now, although the Shuffle works fine, a DIFFERENT photo shows up every time at startup. I want to have the one I choose be the one that always shows up first and stays there until the boot-up is complete. Any ideas? In Sequoia now.