Whether you took the picture or not is not the primary question.
Of course, the primary goal of this site is that it is a place where members show their pictures which they took (correct me if I'm wrong).
But many members including myself are posting pictures which other people took - relatives, friends, ...
The main question is: do you have the copyright?
Even if a picture, print, slide, negative is sold, the copyright remains with the photographer. In my jurisdiction until 70 years after the photographer's death if I remember right.
To post a picture on this site you are required to state the photographer's name and have the copyright of the picture you are going to post.
If you are not sure about this, please don't post!
I have lots of pictures I bought myself which I don't post because the photographer and origin are unclear.
Please see the site standards regarding this topic:http://www.shipspotting.com/support/faq.php?category=Copyright