How to check if image object is the same as one from resource?

So I'm trying to create a simple program that just change the picture in a picture box when it's clicked. I'm using just two pictures at the moment so my code for the picture box click event function looks like that:

private void pictureBox1_Click(object sender, EventArgs e)
    {
       if (pictureBox1.Image == Labirint.Properties.Resources.first)
            pictureBox1.Image = Labirint.Properties.Resources.reitmi;
       else if (pictureBox1.Image == Labirint.Properties.Resources.reitmi)
            pictureBox1.Image = Labirint.Properties.Resources.first;
    }

For some reason the if statement it's not working and the picture don't change. What should I do?

Note: original code contained bug with second if undoing effect of first if condition would work with fix suggested by Cyral's answer, but adding else did not fix the issue - stepping through the code with else still shows no matches for either image.

if (pictureBox1.Image == Labirint.Properties.Resources.first)
    pictureBox1.Image = Labirint.Properties.Resources.reitmi;
if (pictureBox1.Image == Labirint.Properties.Resources.reitmi) // was missing else
    pictureBox1.Image = Labirint.Properties.Resources.first; 

     if (pictureBox1.Image == Labirint.Properties.Resources.first)

There's a trap here that not enough .NET programmers are aware of. Responsible for a lot of programs that run with bloated memory footprints. Using the Labirint.Properties.Resources.xxxx property creates a new image object, it will never match any other image. You need to use the property only once, store the images in a field of your class. Roughly:

    private Image first;
    private Image reitmi;

    public Form1() {
        InitializeComponent();
        first = Labirint.Properties.Resources.first;
        reitmi = Labirint.Properties.Resources.reitmi;
        pictureBox1.Image = first;
    }

And now you can compare them:

    private void pictureBox1_Click(object sender, EventArgs e) {
        if (pictureBox1.Image == first) pictureBox1.Image = reitmi;
        else pictureBox1.Image = first;
    }

And to avoid the memory bloat:

    private void Form1_FormClosed(object sender, FormClosedEventArgs e) {
       first.Dispose();
       reitmi.Dispose();
    }

You need to use else if, because if the image is first, you set it to reitmi, then you check if it is reitmi, which it now is, and change it back to first. This ends up not appearing to change at all.

if (pictureBox1.Image == Labirint.Properties.Resources.first)
    pictureBox1.Image = Labirint.Properties.Resources.reitmi;
else if (pictureBox1.Image == Labirint.Properties.Resources.reitmi)
    pictureBox1.Image = Labirint.Properties.Resources.first;