JP - for me, a screenprint isn't usually the same high res as the original picture on the ebay page. In some ads, ebay has "hidden" the picture name within the html code of the page; some people were "borrowing" pictures of things they don't have, for nefarious purposes.
So ... I just find the "hidden" picture name on the page.
On the page with the picture, use your browser to View Page Source. It's usually under Edit at the top of the browser, or with a right click on the page.
You'll see a new window with all the fun html code. Embedded in that code is the actual name of the picture, usually ending with .jpg.
Copy the whole picture name, which is actually a url within quotation marks. Include the http at the beginning, all the way to the .jpg at the end; then paste it into the address field of the browser.
Voila! You have the original picture now on your browser, so you can save it with the usual "right click to save".
Hope this helps. PM me if you have any question.
Tom