Then it means your messages were set up using Event Espresso decaf and if you are now using the regular version of EE (as you have a license key I’m assuming you are) those paths are no longer valid.
To fix you can go to Event Espresso -> Messages -> Default message templates (tab) -> Receipt.
If you haven’t edited any of that template previously you can just click the ‘Reset Templates’ button on that screen, it will tell EE to regenerate the default template using its current path and should fix the issue with the images.
It should just be text. We just need the information, not a bunch of added images or icons.
You can remove those images from the template if you wish, it’s the page you’ve just loaded above.