SEO for images

Not sure if this has been mentioned before, but I have had a look at my galleries and it appears that each image is given a number:
galleries/female-models/tracy/#gallery-26

Where do find the option to change this, I dont think this is lightroom or is it??

Am a bit confused.

that number is for the slide show presentation only and cannot be changed. Your images still have a permalink that contains the image file name.
To visit the permalink image page, click on the image menu icon in the upper right corner of the large image in the slide show. Then click on the permalink icon (chainlinks). This is the same page used in the Single Image presentation.

Those URLs should not be misconstrued to be permalinks; they are not. That number is assigned on launching the slideshow and is the index of that image within the slideshow array. The first image is 0, counting up from there. If you add images to the gallery ahead of pre-existing images, then the index values will shift accordingly, and that URL would then open whichever image now occupies that position in the sequence of images.

Backlight’s albums provide a permalink for every image which links to that image’s single-HTML page. These links are visible within the slideshow, via the chain-link icon. These links are the links shared to social media, used for SEO, etc. (not the slideshow links!).