Shouldn't you include 'Q_OBJECT'(http://qt-project.org/doc/qt-4.8/signalsandslots.html) here? :
class Myclass : QObject{
Q_OBJECT // <-- here
signal:
void mySignal();
public:
void f();
Image *img;
}
Edit: Generally 'multiple definitions of ...' can be repared with 'static' (ie 'static void f()')
Edit: Did you look here? Multiple definitions error: one in my file and one in the moc file.