How to load the picture file in Web Form

If I click the button, then I want to show the image file behind the button.
that button is invisible after clicking. But I don't know how to implement in Web Form.
This is a part of window form code.
Private Sub btn1_Click(sender As Object, e As EventArgs)
btn1.Tag = 1
buttonClicked(sender)
btn1.BackgroundImage = image(1)
However, there is no tag and backgroundimage event in Web Form.
How to show the image file after click the button control??
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