In your button1_click event, you could add the following
PictureBox1.Image= Image.FromFile("c:\folder\file.gif")
Replace the path with the image you need.
EDIT
To change it everytime, you need to create a global variable 'counter'. Add to that value everytime you click.
Also, create an array of pictures.
So everytime you click, you select the string in the array with the index of counter and you set that as the image in the code presented above.
Dim array() As String = {"c:\folder\file1.gif", "c:\folder\file2.gif", "c:\folder\file3.gif"}
PictureBox1.Image= Image.FromFile(array(counter))