Well, just state the fact like “I approve of this butthole”, or “the artist did a great job at detailing this pubic hair patch”.
But yeah, I totally get it, it is tough.
just being nice is good. keeping the focus on the picture or title and not on yourself as the person commenting is also good. "you have great tits, they look gorgeous!" vs "your tits make my cock so hard and ready to cum on them".
if there's a sexy title that invites sexy responses (like a question or whatever) I don't think there's anything wrong with answering it. just don't go overboard and turn it into a paragraph of sext.
being rude or implying the poster isn't good enough for you is always cringe, unless you're on a kink subreddit dedicated to that kind of thing.
Stop commenting and just whack and move on. Unless the poster specifically wants comments, in which case the usually give you a prompt.
The best way is to do so objectively:
I really like the lighting in this scene!
What great shot composition!
I can see you worked hard on this! Good job!