You can attach documents and images to your invoice. After creating your invoice:

From the invoice listing, click on an invoice to go to the Invoice Details page.

Scroll to the bottom of the invoice and find the “Attachments” section. Then, click the “Attach File” button.
Attachment Section

Next, on the “Upload A File” popup, enter a short description for the file, then click the “Choose File” button to select a file from your system. Finally, click the “Upload” button to upload your file.

NOTE: There’s a 2 MB limit for each file you upload. 
File upload

When your file is uploaded, it will appear in the Attachments listing at the bottom of your invoice. Use the “Add to Email” checkbox if you wish to include the attachment when emailing the invoice to your customer. You can proceed to attach additional files as needed.
Attachment list