A complete example of a button that toggles between two icons in Qt 5.

dialog.cpp

#include "dialog.h"

Dialog::Dialog(QWidget *parent): QDialog(parent)
{
    QVBoxLayout *mainLayout = new QVBoxLayout(this);

    iconBtn = new QPushButton(QIcon(":/icon1.png"), "", this);
    icon = 0;

    mainLayout->addWidget(iconBtn);
    connect(iconBtn, SIGNAL(clicked()), this, SLOT(toggleIcon()));
    setLayout(mainLayout);
}

void Dialog::toggleIcon()
{
    icon = (icon) ? 0 : 1;
    QString fname = QString(":/icon%1.png").arg(icon+1);
    iconBtn->setIcon(QIcon(fname)); //This will set the size automatically
}

Dialog::~Dialog()
{
    delete iconBtn;
}

dialog.h

#ifndef DIALOG_H
#define DIALOG_H

#include <QtWidgets>
#include <QtGui>

class Dialog : public QDialog
{
    Q_OBJECT
    
public:
    Dialog(QWidget *parent = 0);
    ~Dialog();

public slots:
    void toggleIcon();

private:
    QPushButton* iconBtn;
    int icon;
};

#endif // DIALOG_H

main.cpp

#include "dialog.h"
#include <QApplication>

int main(int argc, char *argv[])
{
    Q_INIT_RESOURCE(res); //Required for MSVC 2012.
    QApplication a(argc, argv);
    Dialog w;
    w.show();
    
    return a.exec();
}

res.qrc

<RCC>
    <qresource prefix="/">
        <file>icon1.png</file>
        <file>icon2.png</file>
    </qresource>
</RCC>

TestSO.pro

#-------------------------------------------------
#
# Project created by QtCreator 2013-07-13T19:42:21
#
#-------------------------------------------------

QT       += core gui

greaterThan(QT_MAJOR_VERSION, 4): QT += widgets

TARGET = TestSO
TEMPLATE = app


SOURCES += main.cpp\
        dialog.cpp

HEADERS  += dialog.h

RESOURCES += \
    res.qrc