When creating a new character, I name him/her, create an image of him/her based on web searches, then drag and drop a small version of that image to the synopsis area.
If the Scriv window is not obscured, I drag it to the synopsis area in the inspector, drop it, and it appears where it should.
If windows are obscuring the Scriv window, I drag the image to the Scriv icon on the dock, drag it to the Scriv window, then drag it to the synopsis area in the inspector. In this situation, the image is placed in the document instead of in the synopsis area.
I’ve tested this and it seems to be a bug in macOS. If you move the image around a bit over the synopsis area until you get the green “+” sign, it will work, but the problem is that this won’t work for the whole synopsis area. In this situation, macOS seems to be sending the mouse drag to the wrong area of the screen, so that the synopsis area won’t always see it. The first time I tried, the image ended up in the notes. Unfortunately there’s no way for me to address this since drag and drop on macOS relies on the OS sending the view a “dragging entered” message, and in this case the message isn’t being received as it should.