This question really should be asked in the forum sub-section dedicated to the version (release) of the operating system you're running (for instance, say, Mojave 10.14); this part of the forum is really for MacBook Air specific questions. That being said, the process to accomplish what you desire should be similar for most of the recent releases:
1. Open System Preferences.
2. Desktop and Screen Saver.
3. Choose the style of screen saver you want - Classic for a fixed image, Ken Burns for an image that enlarges slightly and moves, etc.
4. In the right side of the panel there is a Source drop down; the default is landscapes, but one of the choices further down is Choose folder. This is the one you want; you'll need to have a folder setup with multiple images to choose (it doesn't make sense to only have one image in the folder, since that won't really save your screen if you burn that image into your display).
Edit: after reading your question again I think that I misinterpreted it. It appears you want to have the word of the day feature still, but with a custom background? I haven't heard of that being possible.