Pixelfed is a photo and image sharing network on the Fediverse with a photo-oriented interface that includes albums, filters, moments etc. You can follow Pixelfed accounts from Mastodon, and Mastodon accounts from Pixelfed.
You can find out more about Pixelfed including a list of servers to join on its official site at Pixelfed.org ⧉.
There is an official iPhone app in public beta testing on Apple TestFlight ⧉ and a third party Android app called PixelDroid ⧉. You can also use Mastodon apps with PixelFed accounts, as PixelFed is compatible with them.
If you are not technical you can host your own Pixelfed server through managed hosting ⧉, or if you are slightly techy you can use tools like YunoHost ⧉. If you are very techy and just want to install and maintain a server without any help, see the official documentation here ⧉.