Its so simple! You can go with your button click event, I used it with a button property Name: "btnClearImage"
// Note 1a:
// after clearing the picture box
// you can also disable clear button
// by inserting follwoing one line of code:
btnClearImage.Enabled = false
// Note 1b:
// you should set your button Enabled property
// to "False"
// after that you will need to Insert
// the following line to concerned event or button
// that load your image into picturebox1
// code line is as follows:
btnClearImage.Enabled = true;