The easiest way to do this would be to put the image in the ReceiptHeader topic (you can create it if you don't have it, with that exact name). If that topic has anything in it, it displays at the top of the receipt.
You can also edit the receipt XML package directly (notification.receipt.xml.config on most sites) and add in the HTML for displaying the image wherever you want it.
Either way, make sure you use the full absolute URL to the image, and test in multiple mail clients to make sure it looks the way you want. Getting images & other email contents to look the same across all/most mail clients can sometimes be a challenge.