The About post for the Gallery sub-category says very general things …
On the contrary, some people may not be interested in any architectural threads, and only interesting in gadgets that are 3D printed. Having a top level Gallery category and at least a few sub-categories can cut down the multitude of threads if you’re just browsing.
Combination posts could always go in the top level category. Right now the gallery is not so easy to find as a sub category.
But I understand Julia’s point about the challenge of deciding what the sub-categories would be.
I’d suggest not what kind of imagery the post is, but instead the genre of what it’s used for.
Ie, architecture, furniture, 3d prints, etc.
Also users (or later admins and sages) can add tags to threads to aid in filtering.
So Ian’s threads could have all 3 tags: image render layout
Re, Steve’s pseudo categories could map to tags like:
image render layout print3d builds export my1st … etc.