1. I found all sorts of notes, some with just a number such as 2, 1, 27, etc. Do I need to keep them for anything?

2. I've found several black & white real photos hidden away in places. Does anyone know what that's all about?

3. I found a recording talking about a rumor that there are people of short stature deep in the earth, and I found a drawing that looks like a typical dwarf (short, broad, hairy). Anything to that?
edit-got posted twice somehow, sorry.

edit: Thanks to all for the answers. Apparently only one answer can be marked "solved", but I'd do that for all 3 of you if I could.

There's an achievement for collecting all the photos. From what I can tell, the people in those pictures are from Piranha Byte.

The Dwarf reference is an easter egg. In this case, someone found some old pre-comet fantasy books and assumed they're true.
There's quite a bit of those around. I found the skeletal remains of a Pen & Paper RPG group the other day, complete with character sheets and rule book.

In the Gothic games of Piranha Bytes are some references to the "Mighty Alien Dwarf". I'm pretty sure this is just an easter egg, but maybe we see an Addon with Alien Dwarfs that live underground?