ИМХО.
1. В БД правильнее хранить картинку в одном из общепринятых форматов (JPG, GIF, PNG и т.д., а то и в любом из них
).
2. QPixmap - платформо зависимый класс, уж лучше использовать QImage.
Note that the pixel data in a pixmap is internal and is managed by the underlying window system. Pixels can only be accessed through QPainter functions or by converting the QPixmap to a QImage. Depending on the system, QPixmap is stored using a RGB32 or a premultiplied alpha format. If the image has an alpha channel, and if the system allows, the preferred format is premultiplied alpha. Note also that QPixmap, unlike QImage, may be hardware dependent. On X11 and Mac, a QPixmap is stored on the server side while a QImage is stored on the client side (on Windows, these two classes have an equivalent internal representation, i.e. both QImage and QPixmap are stored on the client side and don't use any GDI resources).