Our system only sends thank-you emails in plaintext to help ensure reliable delivery and that they can be read in any email app the recipient may be using, so it's not possible to insert images in these email messages.
However, the link we provide in thank-you emails would take the recipient to your thank-you page where we provide the actual download link, and you CAN add images and other HTML to those pages. Any HTML you put in your Common Thank-you Page HTML field (in Seller Admin > Edit Preferences) would get inserted at the bottom of every thank-you page we generate for you, regardless of the item(s) ordered. Any HTML you put in the Product Thank-you Page HTML field (of an individual item's settings) would get inserted in thank-you page below that item's name and download link only when that item is purchased. This help page provides further details, including an example customized page at the end showing what gets inserted where: