Download any Photoshop template from the gallery
Depending on the template you choose, add the name of your Addon plus additional information or screenshots
Select "Save for web" in Adobe Photoshop and choose png/jpg format
If you have already added your Addon to the Marketplace go to the Control Panel and select the Promotional information section, where you can upload your designs. If you want to add your Addon to the Marketplace follow the tutorial here.