I am guessing that you already figured this out, since you posted 4 months ago, but if not... You are not telling Imagen which edit mode to use, and there is an edit mode that places products on different backgrounds. Use edit_mode="product-image". You provide the base_image (your product) and a description of the background. No mask file required. Details here: https://cloud.google.com/vertex-ai/generative-ai/docs/samples/generativeaionvertexai-imagen-edit-image-product-image?hl=en