mousePressed to change image multiple times

Hi, so I am making a code for a school project. It’s nothing too fancy but I’m very new to the program and have been trying to find a way around the issue but ave had no luck. Basically I have an image that is there when I start the program. What I want to do is when I click the mouse, replace that image with another image. I’ve been trying to do that through the mousePressed function and haven’t been able to, any suggestions?