Hello there, @SquareTriangle.

I can see the importance of adding images to your invoices. You may add the product image manually in the invoice. There's a section on the lower-left where you can attach the image up to 20MB in size. I've added a screenshot for your reference:

However, if you're referring to add the image on each product line, this would be a great idea and I'd recommend sending feedback to our Product developer team. They may include this option in future enhancements. This is where you can send feedback:

I've included this article to show you what are the other transactions where you can attach images: Attachments in QuickBooks Online.

I'll be around if you have other questions or concerns. Feel free to let me know in the comment section. I'd be happy to answer them.