Not sure about explicit support for watermarks but graphics support in QT is EXCELLENT - I don't believe you'll need anything else outside of QT for graphics, or for just about anything else for that matter. It's a pretty comprehensive computing platform and IMO any project is worth doing in QT - it's a great platform.
But if you're delivering commercial software, it's not always openSource - check out the licensing restrictions - you may have to pay for commercial use.
BTW, if you're looking to go cross platform with QT, I'd look at Python-PyQt. PyQt will make your life much easier - and unless you're doing something that really requires C++ 'down to the metal' performance, the performance hit will be neglible on modern hardware - PyQt is mostly a thin wrapper on native QT but without all extra work of C++, and the Python run-time builds compiled native C libs as it executes.
As for the learning curve, if you're an accomplished C++ developer, Python will be a 'piece of cake'.