Hi Robert, see responses below
are you using much descriptive text on your pages? As refined as Google may or may not be able to discriminate among images,
Each image resides on its own page, which has a unique title, description, size and edition options, photo ID, category links, tags, and additional text which is found on each page regarding framing, licensing etc. The description can be made as long as needed, now they are fairly short.
"Regarding infinite scroll, though the scrolling is perhaps useful for browsing for those who don't know what they want, you are effectively eliminating a lot of the structure... aka "pages"... by which a site ranks, ie, page titles, granular descriptive text, and internal nav. I think to a degree you might also be allowing yourself to avoid some of the hard work of classification, though I don't know that.
Google Images doesn't have the same ranking concerns that you do, so I think emulating it is at least partially a mistake. I'd keep the best of both worlds... perhaps allow limited scrolling in your category and subcategory pages, but still maintain the kind of structure you had before"
Yes all the categories and subcategory pages have been ported to the new site, and the scrolling happens only after the visitor navigates to the category. What had been eliminated is the need to click on a series of page links to advance. Sorry my previous response to Aristotle was somewhat unclear in that the cats and subcas remain, although I am paring down some sub subcategories and converting them to tag pages.
"My guess is that the gradual deterioration of ranking you describe is Panda decay over time. I don't think scrolling is going to solve that, unless it helps users find what they want faster than other arrangements. Do you have features like light boxes, etc, which allow users to collect selected images and compare them? That, off the top of my head, is what a stock photo site should be aiming for. Also, the competition in online photography now has become formidable... so your shots have to be very good. I'd be ruthless about editing your photos down to work that is exceptionally good."
The great majority of the loss in traffic was all in one shot, when Google changed image search design to stop referring searches to sites. Image Sites like mine saw a 80% drop in traffic from GI, which was half my total traffic. The rest could be panda but you hit the nail on the head that the number of sites has exploded in the last 6 years. It's just not possible to retain rank across as many categories and subcats as in the past.
I have made an effort to merge all duplicate pages from my old site into single pages with unique urls. The old site had up to 4 duplicate pages for each image entered into multiple categories. I changed each page slightly so always avoided huge penalties, but it's time to restructure and merge. I suspect that redirecting so many pages to one new page might actually concentrate the value and the new unique pages might perform better. I am not making any effort to retain old urls for actual image pages, that would be a massive job to analyze which url performs the best and retain that url intact. If I have time I will do it before the site goes live.
This is not a stock photo site, it's fine art photography which is also sold through partner galleries around the world. The price point is quite high, so a luxury item. The images can be licensed for limited commercial use as well, and a light box has been included in the site. I am also having the devs rework the native WP search feature as that is critical in finding images. A related posts plug in allows me to add linked thumbs to related images throughout the site as well.