Pyqt Qlabel widget for image and mouse event real coordinates

I would like to make a desktop application for annotate annomalies in medical images; basically formed by two Qlabel widgets: the left one for explore the image files (and load one of it) and another Qlabel, the right one, for displaying a ROI of a region defined for mouse events in the image loaded, so when this region be displayed in the 2nd Qlabel you can define the annomalies edges, but the first problem I'm facing is related with getting the real coordinates of the image, I've used QImage for loading the image and QPixmap for showing it in the Qlabel widgets and used mouspressevent for getting the coordinates and conform the rectangular area, but I get a wrong result using the global position (globalPos().x(),...) as the related with the widget (pos().x(),...) so in the Qlabel for showing the ROI I get the wrong area. I've also get the QImage and QPixmap size, which corresponds to real image file size; and the Qlabels size, which varies before an after loading the image in the widget for trying to make some trick scaling the real image size (Qlabel, QPixmap) respect to the modified one (images display in Qlabel) but I'm new in python and I'm not advancing as I thought Qt would let me... Please can someone advice me with the kowhow in a problem with this characteristics... Thanks a lot for your time…greetings