Images are added on the product attribute. You can then decide on each quote line whether or not to display the image.
- Go to Quote > Edit Sell Products
- Scroll down to the last section on the page Product Attributes
- Click on the name of the attribute and select Edit attribute name

- Click in the checkbox for Attach an image to each value and click OK
- Click on the name of the attribute and select Edit List of Values

- Click on Edit to the right of each value and and attach an image

Now when you add the product to a quote, you'll see the image on the dialog box and have the option to display the image on your quote. Click the checkbox for Display Images.