Question: I have the current situation:

in iPhoto, I can see some of my pictures made in the last months ONLY in Photostream. iPhoto says they're imported already, but "show in Library" leads to nowhere.
Since Photostream has a 1000 pictures limit, I'm afraid I'll soon lose some photos I really like.

Last import is supposed to have deleted all pictures from the iPhone, so these pictures are only in Photostream... does anyone have an idea what to do to have these pictures show up in my Events, as they should be?

Answer: 
You must press `command` + `option` while dragging.
Hold down  `cmd`  and  `option`  when dragging.