I'm not sure what prompt you put in but did Gemini replace the all of the images in the original with its own? That would be really weird behavior unprompted.
https://image.non.io/12275ee8-71e9-4941-823b-e51fec157b4d.we...
The agent is told to generate assets as part of the buildout. It gets to decide what the prompt is for them / whether to do postprocessing like background removal / what type of asset to generate.