How about Lensdump. Doesn't have a gallery, you just share the URL, but the nice thing is there's an auto-delete feature. Guess that would more handy if you were sharing pictures to friends/family, and not so much a forum/board, but still a nice feature to have.
Fediverse
This magazine is dedicated to discussions on the federated social networking ecosystem, which includes decentralized and open-source social media platforms. Whether you are a user, developer, or simply interested in the concept of decentralized social media, this is the place for you. Here you can share your knowledge, ask questions, and engage in discussions on topics such as the benefits and challenges of decentralized social media, new and existing federated platforms, and more. From the latest developments and trends to ethical considerations and the future of federated social media, this category covers a wide range of topics related to the Fediverse.
Sorry for necrobumping this thread and you may have already found a solution, but you might try Picsur. It's an open-source, self-hosted Imgur clone. The self-hosted aspect might not be exactly what you're looking for, but I did set it up in docker and it worked well and was a simple process.
The development seems to have slowed down a bit, but it does seem feature complete.
I don't understand. What's the point of posting private photos to a federated service? Just use Imgur.
"When all you have is a hammer, every problem looks like a nail."
Imgur announced automated image blocking, no NSFW private images, plus purging of non-account images. None of it affected me, but it breaks stuff on the internet.