conversion of x and y coordinates to latitude and longitude

sir ,

Actually i have a gui which contains qcustomplot widget with a map inside the widget. if i click on the widget am able to get the cursor position on the map with x and y values . now i need to convert that x and y coordinates into latitude and longitude .
presently am using qt4 . can any body suggest me how to do this . my specific request is to suggest me the solution at the earliest

If you know the origin position and size of the image in pixels and the start and end longitudes and latitudes of the map, then this is an easy algebra problem.

I think the next article can help you - how-to-convert-x-y-coordinates-to-longitude-latitude-using-quantumgis