i think both work to the point that no one would notice the difference.

for example if you also had a "video gallery" it'd most likely be just that. "video gallery". as opposed to "videos gallery".

and so accompanied by the singular form of "video", "still" wouldn't look outta place at all. the consistency would trigger the connection.

but if you had "videos" gallery then it's referencing each bit of material within the gallery as opposed to the gallery itself, and so you'd want "stills" to go with it.

and so OMFG OK MY BRAIN HURTS I AM SO DONE WITH THIS THREAD!