Hi Romain,
Just to clarify, this is using "AEM Mobile" (AEMM) right? AEM is a different product. From your description, it sounds like it could be AEM Mobile, but there has been confusion in the past.
I have not heard of images lowering their resolution when not connected to WiFi (or Data).
This would take some investigation and more details (Publication Id, etc). I suggest you call support and log an official issue so people can allocate some time to investigate this problem.
Andrew