How do I add more custom photos to a listing?

First, you need to add your custom photos. To do this, click on My Images in the MyBrickLink > About Me tabs. Each photo will be assigned an ID number for HTML tags. Here is what it looks like:
Then, visit the My Inventory page through the Sell tab. Click on the item you wish to add more photos for. You will see a link that says "Edit More" which will take you to a page where you can add the HTML tags for additional photos. The HTML code is like this: "<IMG SRC=""> where "PHOTOIDNUMBER" is the number assigned to each photo in My Images. 
See below screenshot for reference:
In the above screenshot, I have replaced PHOTOIDNUMBER with 248259.jpg, the ID number of the photo.
Now when you visit your store, your listing will look like this: