If you're using a photo or graphic from another website for discussion purposes, then copyright in most cases is not the issue when protected under fair use doctrine. Though appropriate to properly credit the author. However, fair use does not cover against stealing server bandwidth without permission. Which is what happens when one embeds the photo from the owner's website. Use a separate photo hosting instead.
To answer the original question, some websites use Javascript to disable right-click in order to block copying URL. A weak solution, as disabling Javascript defeats the ability to do so. However, all major browsers have developer mode, usually the F12 key, which would allow you to determine the URL of the photo. Firefox also has right click on the page to View Page Info. In the Media tab, you can find the photo URL or download in most cases.
