Qt symbol(s) not found for architecture x86_64 - c++

Question

First of all I am new to Qt, so if there may be something painstakingly obvious to some more advanced users, I also know this error crops up a lot and I've done a lot of reading on Qt forums and have still not managed to solve my problem.
I don't however understand why I am even getting the symbols not found for architecture x86_64 error as I had been working on my application all day without any problems.
I am building an image editor in the likes of Photoshop for an assignment in my Cross Platform SW Development course at Uni and wanted to separate the filter logic I was writing from the rest of my code, I therefore created a Filters class.
In my mainwindow.h I created a private reference to the class Filters *filter; which I instantiated in mainwindow.cpp
MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
// Instanciate a scene object that we can draw to and then set up the ui
scene = new QGraphicsScene(this);
filter = new Filters(this);
ui->setupUi(this);
QObject::connect(ui->cmdBlack_White, SIGNAL(triggered()), filter, SLOT(serialFilterBlackWhite(image)));
}
As you can see, I have created a connection to the serialFilterBlackWhite method, which resides in filters.cpp at the moment and doesn't do much due to the x86 error:
QPixmap Filters::serialFilterBlackWhite(QPixmap image)
{
image.fill( Qt::red );
return image;
}
The .h file for the class looks like this :
#ifndef FILTERS_H
#define FILTERS_H
#include <QObject>
#include <QPixmap>
class Filters : public QObject
{
Q_OBJECT
public:
explicit Filters(QWidget *parent = 0);
signals:
public slots:
QPixmap serialFilterBlackWhite(QPixmap);
};
#endif // FILTERS_H
I have tried not too bloat my question with too much code, but if you need more please let me know. I really have no idea whats going on with this.