The images that we can insert into an Online Express confirmation email will need to exist online as we aren't able to upload a .jpg file, for example, into the email. We can use the following steps.
1. Go to any page on the Internet where the image you want to use is present.
2. Right-click on the image and select "Copy image" in the pop-up menu.
3. Go into Online Express and just paste (Control+V) the image on the Thank Yous tab of the Donation Form.
Developers are working on other options for adding images to these emails.