Thank you for clarifying your concern, @RKR29061.
At this time, QuickBooks Online (QBO) only allows you the attach documents on invoices.
I can see that the option to add a thumbnail to each line item on an invoice is beneficial to you and your company to provide the best for your clients. But also know that we are taking notes of our clients' suggestions as well as yours to improve your experience in QuickBooks.
For now, I encourage sending feedback to our engineering team by going to the Gear icon, then selecting Feedback. This way, they can consider checking and working on the option to add a thumbnail on an invoice.
Also, you can visit our blog site so you'll be able to get the latest news about QuickBooks and what our Product Care Team is working on.
Please keep in touch if there's anything else I can do to help you succeed with QuickBooks. I've got your back.