Do you need an easy way for your customers to tell you which photos they're talking about? If you're using SmugMug-style galleries, the photo has a number written underneath it (like "4/49") which is useful for identification. But in Collage Landscape, Collage Portrait, Journal and Thumbnail gallery styles, this number is absent.
This customisation adds a number to each photo thumbnail. Unfortunately, it isn't possible to display that number in the Lightbox view that appears when you click on a photo.
To apply this effect to all your galleries, add this code to your theme's CSS. To apply it to just a couple of galleries, add this code as a CSS content block in those galleries.
If you only want the numbers to appear when you hover over each thumbnail, add this CSS to the bottom: