Why is gallery my only choice for photo sharing through apps like Instagram and Facebook?

[Android Central Question]

This is frustrating. I want to share a photo in from within Facebook, like a new status. The only place I can share from is gallery. It has a very limited selection of my photos. I want to use Google photos where I can see all my photos.
I get the impression the gallery photos I'm seeing are from something called timeline. I have no idea how to manually populate the gallery and I don't want to have to. Same for Instagram. Isn't there a way to set the default photo manger app?