Replacement of the background on serial photos from white to gray circle

There are photos of light objects (14 pcs) on a practically white background.
Each subject - 12 photos in different angles.
It is necessary to replace the background with a new one - a gray circle on white.
An example of the original and processed photo is attached.
It's important! Photos are shown in a script sequentially, so the size, color and position of the gray background in the series are the same; images in a sequential display (such as a slide show) should not “jump” in geometry, colors, or brightness, etc.