This a very tricky issue.
There’s no Discourse feature that would fit with this, and even less since only a certain number of pictures are allowed in one post.
I suppose I would create a category with subcategories and/or tags.
Category: https://www2.sidemount-forum.com/gallery
Subcategory: https://www2.sidemount-forum.com/gallery/album/133
Topic: https://www2.sidemount-forum.com/gallery/album/134 (each post would contain 1 image)
I have no other idea right now 
I’ll also move your topic to Support > Migration since the issue is basically how to port the data from a feature from your old platform to Discourse.