If you are looking to add the Image of the products in the invoice, You have reached the right spot, just follow the steps given below:
Step 1: Click on Products & Services >> add new item >> Upload Image >> Add item
Step 2: Click on Document setting >> ensure Show image is turned on
Step 3: Add the product and save >> View PDF
Vola ! you have successfully added a image to the product
Step 1
Step 2
Step 3