Thanks for the quick response. I see that the image now has "_thumb" appended to the name but my issue is that my thumbnails are located in another directory under the main gallery. So, for instance, my main images are located in /gallery/web-design/ and my thumbnails are located in /gallery/web-design/thumbs/.
Additionally, my thumbnails are named as "thumbs_XXXXXXX.jpg"
Sorry, it appears that by default, Nivo searches for ".jpg" and replaces it with "_thumb.jpg" so I'm not sure how to modify that to prepend the image with "thumbs_" since there's nothing to search for and replace as it's a prepend.
The web inspector, however, still shows that the thumbnails are not being looked for in /gallery/web-design/thumbs/.