In Confirmation/Notification Settings, you can insert images when using a Custom Message.
Go to Forms -> Actions -> Settings > Confirmation Settings / Notification Settings pages and enables the email message. To insert an image into your custom message email, click on the Picture button (which looks like a small mountain).
This will display the image box. In “Source” enter the absolute URL to the file.
Once the url is added, you will have options to resize and enter an alternative description.