Hello Mario,
Thank you for reaching out. The AdGroupAdService will return the ImageAd via API if it exists in your Google Ads account which will contain the mediaId of the image. Could you please share the complete request and response logs along with the client customer id for me to investigate this further on my end and assist you better?
P.S. Please use Reply privately to author option while sharing the details.
Regards,
Nikisha Patel, Google Ads API Team