You missed the time of duration of the Image Inserted. Like 20 secconds. for example:

"InsertableImages": [ { "Width": 100, "Height": 31, "ImageX": 0, "ImageY": 0, "Layer": 20, "ImageInserterInput": "s3://project-profile-account_id-us-east-1/watermark.png", "StartTime": "00:00:00:00", "Opacity": 50, "Duration": 20000 /This value/ }