Pixelfed is a photo sharing network on the Fediverse, intended as an alternative to Instagram. You can follow Pixelfed accounts from Mastodon and vice versa. There's more info about it at:
I’ve been using it for around 3 weeks and been having a great time. The home feed is so much cleaner than Instagram, especially without all the ads. Find a bunch of cool people to follow and enjoy!
I can also recommend the Vernissage app if you want a ultra clean feed.