The Venom Skin is a Marvel Fortnite Outfit. This cosmetic has been leaked, and the skin will hopefully be available to purchase in the near future!
Venom has been teased via leaks in Fortnite through a power you'll be able to collect and an achievement (Legacy) you will be able to earn because of it. This alone doesn't confirm that there will be a Venom skin, but it does mean it's a likelihood. The image shown above is an image from a comic book, not what it will look like in-game necessarily.
How-to Get the Venom Skin
Venom was leaked or datamined, which means we don't know for sure how it will be released. Most cosmetics that are leaked will be released within 30 - 60 days of their discovery.