我认为这个查询显示了您想要的：

select * from PictureGalleries
    join Followers on Followers.userId = PictureGalleries.UserId
    where exists (select 1 from Friends where (Friends.senderId = PictureGalleries.UserId or Friends.receiverId  = PictureGalleries.UserId) and accepted = 1)
    and Followers.UserIsFollowing  = :user_id

但我认为你的模式可以改进
编辑：
也许你先说错了当你说：
“其中userid与userid 12有友谊关系，其中accepted为1，并且userid 12跟随特定用户”
我想你的意思是，或者，sql应该是这样的：

select * from PictureGalleries
where exists (select 1 from Friends where (Friends.senderId = PictureGalleries.UserId or Friends.receiverId  = PictureGalleries.UserId) and accepted = 1)
OR exists (select 1 from Followers where Followers.userId = PictureGalleries.UserId and Followers.UserIsFollowing  = :user_id

使用union和sub查询获得所需的结果

select p.id,p.UserId from
   ( select UserIsFollowing as id from 
   Followers fl where userId =12 
   union select senderId from friends f 
   where f.receiverId =12 AND accepted=1 
   union select receiverId from friends f 
    where f.senderId =12 AND accepted=1 
   ) as t join PictureGalleries p on t.id=p.UserId