SEO and visibility:hidden

I want to add a description for the images I have on my lightbox galleries, so I have better results on Google.
But I don’t want to show the captions, I thought I would use visibility:hidden, but I am not sure how this affects SEO ?

Well, I’ve just realised that I could simply use Alt instead, and not Caption…
I didn’t see it for some reason !

Anyway, I am still curious to see how this works, if anyone has a good article about it