Hi there, @mattm.

To attach an image to an estimate or an invoice in QuickBooks Online is very easy. Please follow these steps below.

1. Select the Plus icon.
2. Choose Estimate or an Invoice.
3. Enter the needed information.
4. At the bottom left of the page, select Attachments.
5. Browse and select the file you want to attach, then click to open.
6. Select to Save and send.

You can also add or attach an image to any transactions you have. Here’s how:

1.     Open the desired transaction.
2.     From the bottom left, select Attachments.
3.     Browse and select the document you want to attach.
4.     Select Open, then Save and close.

That's it. You now know how to add attachments in QuickBooks Online. You can also check this article for additional information on how to view all uploaded attachments:Attachments in QuickBooks Online.

Don’t hesitate to reach us again if you need additional assistance. I’ll be here anytime. Have a great day!