I WOULD LIKE TO ADD A LANDSCAPE PICTURE TO IMAC

Simple, find the picture you want either in the desktop pictures provided by the OS or from whatever source you want, internet, personal pics, etc. If pic is from outside of desktop pics then put it in Pictures folder then go to Desktop and Screen Saver in System Preference files. Then go to desktop(top of window) then scroll down to folders then Pictures,

then find your chosen pic and set it as desktop. Or right click on the pic you have chosen and go to bottom of window to services and go down to set desktop picture and click. Be sure to keep pic so if something happens you can re-install it as Desktop pic.