==> main.cpp <==
#include "raytracermainwindow.h"
#include <QApplication>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    RaytracerMainWindow w;
    w.show();

    return a.exec();
}

==> raytracermainwindow.cpp <==
#include "raytracermainwindow.h"
#include "ui_raytracermainwindow.h"
#include "renderdialog.h"

#include <QMessageBox>

RaytracerMainWindow::RaytracerMainWindow(QWidget *parent):
    QMainWindow(parent),
    ui(new Ui::RaytracerMainWindow)
{
    ui->setupUi(this);
    raytracerRenderDialog= new RaytracerRenderDialog(parent);
}

RaytracerMainWindow::~RaytracerMainWindow()
{
    delete raytracerRenderDialog;
    delete ui;
}

void RaytracerMainWindow::on_actionShow_render_triggered(bool checked)
{
    if(false== checked){
        raytracerRenderDialog->hide();
    }else{
        raytracerRenderDialog->show();
    }
}

void RaytracerMainWindow::on_actionRender_triggered()
{
    QImage renderedImage;
    renderedImage.load("Tux.png");

    raytracerRenderDialog->renderedImageContainer->setPixmap(QPixmap::fromImage(renderedImage));

    ui->actionShow_render->setChecked(true);
}

==> raytracerrenderdialog.cpp <==
#include "raytracerrenderdialog.h"

RaytracerRenderDialog::RaytracerRenderDialog(QWidget *parent):
    QDialog(parent),
    renderDialog(new Ui::RenderDialog)
{
    renderDialog->setupUi(this);
}

==> renderdialog.cpp <==
#include "renderdialog.h"
#include "ui_renderdialog.h"

RenderDialog::RenderDialog(QWidget *parent) :
    QDialog(parent),
    ui(new Ui::RenderDialog)
{
    ui->setupUi(this);
}

RenderDialog::~RenderDialog()
{
    delete ui;
}

==> raytracermainwindow.h <==
#ifndef RAYTRACERMAINWINDOW_H
#define RAYTRACERMAINWINDOW_H

#include <QMainWindow>
#include "raytracerrenderdialog.h"

namespace Ui {
class RaytracerMainWindow;
}

class RaytracerMainWindow : public QMainWindow
{
    Q_OBJECT

public:
    explicit RaytracerMainWindow(QWidget *parent = 0);
    ~RaytracerMainWindow();

private slots:
    void on_actionShow_render_triggered(bool checked);

    void on_actionRender_triggered();

private:
    Ui::RaytracerMainWindow *ui;
    RaytracerRenderDialog *raytracerRenderDialog;
};

#endif // RAYTRACERMAINWINDOW_H

==> raytracerrenderdialog.h <==
#ifndef RAYTRACERRENDERDIALOG_H
#define RAYTRACERRENDERDIALOG_H

#include "ui_renderdialog.h"

class RaytracerRenderDialog : public QDialog, public Ui::RenderDialog
{
public:
    RaytracerRenderDialog(QWidget *parent = 0);
private:
    Ui::RenderDialog *renderDialog;
};

#endif // RAYTRACERRENDERDIALOG_H

==> renderdialog.h <==
#ifndef RENDERDIALOG_H
#define RENDERDIALOG_H

#include <QDialog>

namespace Ui {
class RenderDialog;
}

class RenderDialog : public QDialog
{
    Q_OBJECT

public:
    explicit RenderDialog(QWidget *parent = 0);
    ~RenderDialog();

private:
    Ui::RenderDialog *ui;
};

#endif // RENDERDIALOG_H