Changing my desktop on my new laptop troubles- Acer Aspire One

[Troubles5050]

I am looking for a way to change my desktop on my Acer Aspire One laptop that I got for my birthday, I have tried at least 50 times with the same picture and nothing I have tried has worked! It is making me rather mad that I can't figure it out. If you know how to do this can you please inform me

 

[USAFRet]

What OS?