How to add an image or icon to the button
Go to block section and click on button, here you can add icon with button from material icon/picture/Font Awesome.
To add image to the button, first upload the image in assets you want to show in button.
Then in button properties there is option to select image. Select the uploaded image and adjust its size.
To add icon to a button, instead of button take a label set as clickable and in properties of label select fonts to material icons, and rename the label with material icon name.