I don't think it makes much difference whether you use QAbstractScrollArea or QScrollArea. QScrollArea does not add much to the core functionality of QAbstractScrollArea.

However, it you are developing a paint-style application, I do not think you want to use a QLabel. QLabel is designed pretty much for non-interactive display of text and images. Since your users will be interacting with the image, you probably want to implement your own QWidget-based class where you have control over what happens in response to mouse, keyboard, paint, and other events.

If you haven't done so already, be sure to look at Qt's Painting Examples.