Just because a site is image or media heavy doesn't mean it can't rank -- or that you can't enhance that media by SHOCK DESCRIBING IT.
Let's find us an example... Google "Porn Videos" and the first result is xvideos.com -- so they must be doing something right.
Assuming you are old enough, you look at their site and while it's a bit of an accessibility wreck (fixed layout, fixed fonts) they have something VERY important -- TEXT inside those anchors describing what each and every video on the page 'is'. There's PLENTY of text on that page and most every major link has text inside it -- DESPITE the page being mostly about thumbnails and links to the videos.
You pick a video, they have a nice big heading, a text list of 'tags' for the video, etc, etc...
There's a reason both porn and non-porn sites operate in this manner; again see one of the major success stories of the Internet, YouTube. For a site that has its entire focus being the delivery of video, there's a HELL of a lot of text there describing what those videos ARE.
Even one sentence for a gallery of thumbs can be all you need; google "Free porn pics" and the first listed site, milfPorn.net follows this pattern... It may only have one word per link like "Mom", "Lingerie" or "HouseWife" -- but if that ONE word is what people are searching for, that's ALL you need to have search pick you out of the crowd!
The redundant lists at the bottom helping too, as does that smallish paragraph at the top of the page.
... that's the way you do it.
Laughably in terms of accessibility and code, improvements to those sites could further distance them from the competition -- or even more of a laugh give someone a legitimate chance at knocking them down a peg.