Why won't my wallpaper show on the projector on presentations.

Hi, so i have a question. Everytime i plug the adapter from my macbook pro to the slide projector, it turna on, but when the pictures are coming together, it didn't show my wallpaper on the screen, instead it shows the galaxy wallpaper, you know, the first wallpaper thats installed when you buy your mac. I changed my wallpaper a few times but it didnt work, it still shows the galaxy wallpaper. My friends' macs shows their wallpapers, but mine didn't, whys that? please tell me how to fix this. thank you 🙂