Hi Morry, welcome aboard.
Unfortunately we don't know which instructions you followed. But in principle you're right: In front is your video source with the green-screen-technique, then the background image behind.
If you say that the image disappears as you put it behind the video source, that just means, that your chroma-key filter (needed for the green-screen thing) doesn't work (yet) the right way. A short summarize then: The chroma-key must be in the filter chain of the video source, because it shall make parts of the video content just transparent. Go into the video sources' filters list and check the chroma-key filters properties. There you will find a handful knobs (sliders) to carefully adapt to your needs. There is an preview too, i think, so you may check live, if the background/green screen will fade away and make alpha/transparency room for your background image. On those parts it should come thru then.
If that wasn't clearly described in your instructions source, look for another youtube videos. There are a lot of people showing this.
(A sidenote: Although windows calls programs meanwhile an "app" for years what establishes a kind of "belittlement" and "being light-weighted" to all serious software, you will see that OBS is a community driven and professional conduct of a live production software program. :D )
