I have done this before, a bare bones example can be done easily in Lambda with GraphicsMagick and ImageMagick for node https://www.npmjs.com/package/gm see "annotate an image" example.
I understand your service does more that this. I map the POST JSON to GM params to allow the caller to do nearly anything GM CLI can do.
I understand your service does more that this. I map the POST JSON to GM params to allow the caller to do nearly anything GM CLI can do.