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. You can select the image to upload the file or you will also have the option to display it from a link.
Once the image is added, you will have options to resize, align and more. These features can be accessed by clicking on the Image.