We have a large collection of image slideshows hosted on site categorised in different sections. We are looking to revamp the site to make it more SEO friendly. The photogallery architecture will need a overhaul. As the main issue currently is crawlibitly, and there is huge content archive that we have. Suggestions on architecture will help.
The trouble with a big photo gallery is that you may simply not have the time to create unique content to accompany each image.
My advice on your approach would depend largely on this. If you have unique descriptions for each image, then each image should have its own separate HTML page with a unique URL. This has a number of advantages, including that people tend to "like" and share individual pictures, so giving each one its own URL helps with that.
If you don't have descriptive text for each image, I would put the images on category pages and use some sort of JS interface (e.g. lightbox) to display larger versions.
Either way, the most important thing is to organize the images into clear, well-accepted categories according to what people are searching for. The categories will need to have unique descriptive text, whether you have unique text for each image or not.
Be careful as a lot of gallery apps inadvertently spawn duplicate content with cross categorization. You can also get minor duplicate content issues if the photo/description appears on a solo page in addition to a master gallery page. If the gallery has a search function make sure that doesn't get crawled as well.