A fish pic refers to the common phenomenon of men posting photos, primarily on dating apps, of themselves holding excessively large fish that they have caught. The PG equivalent of a dick pic, the fish pic is designed to highlight masculinity and compensate for lack of personality.
I'm totally left-swiping any guy on Tinder with a fish pic.

I'm so boring, good thing I have a fish pic so girls think I'm masculine!
by hl189980 September 4, 2015
A Fish Pic is a picture you upload onto a social media site in order to get a lot of compliments.
Girl - I don't feel good about myself, let me put on some makeup and pose for a selfie so I get all the likes
<Uploads Fish Pic>
Girl: Yay I feel good now.
---Time Goes By----
Girl - I don't feel as hot as I did 2 days/months/years ago and that makes me feel bad, let me upload an old fish pic to feel better
<Re-uploads Fish Pic>
Girl: This always works! I'm never going to stop uploading Fish Pics.
---Strategic Time Later---
Girl: I've just added that Tinder guy on Facebook and I need him to know I'm popular.
<Uploads Fish Pic>
Girl: He'll totally want me now, even more than... No I never think this far, I LOVE FISH PICS.
-----Getting ready for a random event when you realize how fucking hot you look-----
Girl: ... oh my god, Fish Pics.
<Takes a million selfies to use as Fish Pics later>
by James von Bonbons December 24, 2014