This is a Paid Feature. To upgrade your account visit our pricing page.
You can add a thumbnail image to your button. This applies to only Link (Plain) & Mailchimp content types.
To do this, head over to your page and click the Links tab if you aren't already on the Links & Content portion of your page.
Open the link settings by clicking any text or the 3 vertical buttons.
Make sure Thumbnail Image is turned on by clicking on it. Once it's on, you can follow the dialogs to upload an image.
You will be able to crop this image and save it.