Hi Melbourn.
Welcome to the HP Forums, I hope you enjoy your experience! To help you get the most out of the HP Forums I would like to direct your attention to the HP Forums Guide First Time Here? Learn How to Post and More.
I see that you are attempting to print envelopes. Here is a link to the HP Deskjet 1510 and Deskjet Ink Advantage 1510 All-in-One Printer Series Product Specifications that shows the supported envelope size.
I have also found a great document that shows Loading Paper, Photo Paper, and Envelopes.
If you have any other questions or concerns, please feel free to write me back.
Thank you for your time.