Make sure you are familiar with the basics of adding and designing a graphics overlay in Studio.
Your guest should set up their shot so they are in front of a solid color, such as a green screen. They should be sure to not wear the same color as their background.
1. In the Guests tab by clicking Add As GFX which creates a default gallery view of one local video source and all of your remote guests as a graphic, as discussed here.
2. In the Guests tab, select Add As Input underneath your guest's camera shot, then head to any GFX tab, create a custom layer, and add your guest's input in the GFX Designer manually, similar to a picture-in-picture. This is a better option if you want no other guests or video sources in the graphic.
In the GFX Designer, select the guest with the green screen, then click the attached chroma icon to key out their background color and give them a transparent background.
From there, you can import a static image into the GFX Designer. Use the scale, crop, and drag-and-drop functions on both the guest and the image to fully layer the guest's shot on top of it. If the background image is overlapping the guest, select the image, then right-click on it and select To Bottom Layer to bury it under the guest's camera input (and anything else in the graphic).
To show your guest, select the appropriate GFX in the multi-view to put them into Preview and use the transition controls to switch them into the Program Output as you would any other source.