There isn't a way around it but there is a way to make in almost invisible.
DK will hate me for this but I don't think many users will bother trying it.
Anyhoo, just upload a REALLY big image, as big as the system allows.
The watermark is placed AT SIZE on the original file but when the webpage shrinks it you will hardly notice it.
This example is almost big enough, as uploaded by our illustrious leader.