How do you post things to Instagram, Imgur, or Flickr without losing
one's copyright?

I don't know that anything I have is worth anything financially but I do
want the credit. My name or my father's name accompanying every
properly used use of the photo.

Right now, two things are on the horizon. I'm transcribing my father's
trip diary from a trip he took in 1936, and I'm scanning the photos he
took on the trip.

I'm his only surviving heir. And my mother left everything he left her
to me. Does that mean I myself have any rights regarding these things?

Even if I have no rights to the pictures, since no one can get into my
house and copy them, I have the vague idea that I'd have rights to the
scanned copy I plan to make of them, for posting online, and that those
rights would last as long and be as strong as if I'd taken the pictures
myself. Am I right at all?

And how do I post these pictures and the separate text to Instagram,
Imgur, or Flickr without losing my copyrights?

Thanks