Sendero Social's PixelFed Instance
PixelFed is an image sharing platform, an ethical alternative to centralized platforms.
Based on the ActivityPub standard, PixelFed allows you to interact with users on any server or application that is a part of the Fediverse.
The best way to use PixelFed on iPhone is to navigate to the site, and then click "Add to Home Screen." The site is optimized for mobile. On Android, there are apps available including FediLab and PixelDroid.
If you are interested in a more feature-rich social networking application, check out Sendero Social's instance of Friendica at sendero.social. Users can interact across the platforms, and are welcome to create accounts on one or both applications.