Maybe the best place to post photos on the Fediverse is PixelFed.
✅ PixelFed is the Fediverse's alternative to Instagram, with a similar interface and features. You can find out more at https://www.pixelfed.org and there's a list of instances to join at https://beta.joinpixelfed.org
It doesn't matter which instance you join, people on other PixelFed or Mastodon instances can follow you because Fediverse instances talk to each other.
Sorry, I made a mistake with the URL, the address is https://pixelfed.org
(which will, strangely, look identical on Mastodon but isn't the same address, it has no www 😕 )