I want my app to be able to manipulate images and create watermarks and also needs to run on both windows and mac. Hence I've been investigating QT.
Does QT ship with all the libraries I will need to achieve that? or will I have to get other standard C++ libraries? If so is it worth doing the project in QT?
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'.
