To set a custom desktop background wallpaper on Mac OS X Yosemite using AppleScript, you can use the following code:

tell application "System Events"
    tell process "System Events"
        set name of every window to "Desktop"
        set background color of every window to {0.5, 0.7, 0.9}
    end tell
end tell

set desktop picture to POSIX file "/path/to/your/wallpaper.jpg"
tell application "System Events"
    tell process "System Events"
        set name of every window to "Desktop"
        set background picture of every window to desktop picture
    end tell
end tell

Replace /path/to/your/wallpaper.jpg with the actual path to your desired wallpaper image.

This script sets a solid color background and then sets the wallpaper image. You can modify the background color line to use a different color or gradient if you prefer.

Note: This code is specific to Mac OS X Yosemite, so make sure to test it on that version before using it on other versions of macOS.