Showing .png on button press

So I’ve looked around and can’t find anything that explains this so here we go:

I’ve got my game and I’ve set up several different buttons and I need them to show a .png file when the button is pressed but I’d like to have a different image depending on weather you left click or right click.

The main problem is I don’t know how to show a .png file when the buttons pressed.

Any help is appreciated.
Thanks in advanced.

Texture2D image
Texture2D image1
Texture2D image2
bool showImage

if the button is clicked (left mouse button)
→ showImage = true
→ image = image1

if the button is clicked ((right mouse button)
→ showImage = true;
→ image = image2

if showImage = true
→ GUI.DrawTexture(image)