When you log into an account on your Mac with Yosemite, you’ll see a fuzzy background behind the login prompt. This image is at /Library/Caches/com.apple.desktop.admin.png. You can replace this image with any png you like. Note that the image is shown with reduced brightness, so some experimentation will be needed if you want to get it just right.
MacMost Online Courses
Courses at Udemy