Form.H

#ifndef FORM_H
#define FORM_H

#include <QWidget>
#include "test.h"

namespace Ui {
class Form;
}

class Form : public QWidget
{
    Q_OBJECT

public:
    explicit Form(QWidget *parent = nullptr);
    ~Form();


signals:
    void valueChanged(int newValue);

private slots:
    void on_button_clicked();

    void off_button_clicked();

private:
    Ui::Form *ui;
    int value;
    Test obj;
};

#endif // FORM_H

Test.h

#ifndef TEST_H
#define TEST_H

#include <QtCore/qglobal.h>
#if QT_VERSION >= 0x050000
#include <QtWidgets/QWidget>
#else
#include <QtGui/QWidget>
#endif

class Test : public QWidget
{
    Q_OBJECT

    public:
        explicit Test(QWidget *parent = nullptr);
        void EnableImage(int value);

signals:
        void ValueChanged(int value);
};

#endif // TEST_H

widget.h

#ifndef WIDGET_H
#define WIDGET_H

#include <QWidget>
#include <QLabel>


namespace Ui {
class Widget;
}

class Widget : public QWidget
{
    Q_OBJECT



public:
    explicit Widget(QWidget *parent = nullptr);
    ~Widget();

    QLabel *Picture;

public slots:
    void setValue(int value);

public:
    Ui::Widget *ui;
};

#endif // WIDGET_H

form.cpp

#include "form.h"
#include "ui_form.h"

Form::Form(QWidget *parent) :
    QWidget(parent),
    ui(new Ui::Form)
{
    ui->setupUi(this);
    value = 0;
    Test obj;
}

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

void Form::on_button_clicked()
{
    value = 1;
    obj.EnableImage(value);
}

void Form::off_button_clicked()
{
    value = 0;
    obj.EnableImage(value);
}

main.cpp

#include "widget.h"
#include "form.h"
#include "test.h"
#include <QApplication>
#include <QObject>


extern Widget w;
int main(int argc, char *argv[])
{
    QApplication a(argc, argv);

    Widget w;
    Form f;
    Test t;


    w.show();
    f.show();

    // help show windows apart
    f.move(w.geometry().left() + 50,w.geometry().top() + 50);

    QObject::connect(&t, SIGNAL(valueChanged(int)),
                         &w, SLOT(setValue(int)));


    return a.exec();
}

test.cpp

#include "test.h"

Test::Test(QWidget *parent) : QWidget(parent)
{

}

void Test::EnableImage(int value)
{
    emit ValueChanged(value);
}

widget.cpp
#include "widget.h"
#include "ui_widget.h"

Widget::Widget(QWidget *parent) :
    QWidget(parent),
    ui(new Ui::Widget)
{
    ui->setupUi(this);
    Picture = ui->image;
    Picture->hide();

}

void Widget::setValue(int value)
{
    if(value == 0)
    {
        Picture->hide();
    }
    else
    {
        Picture->show();
    }
}

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