Upon had been used for all of those purposes in the past, and it can be used for more literary writing or poetry, but now 'on' , 'of', or 'about' is most often used instead for normal speech and writing. 

correct use: ( in parentheses is the modern word or phrase)

I think upon (about/of) him when I look at his picture.

She sat upon (on) the bench and ate her lunch.

Once upon a time.... ( A long time ago... )

I don't really know Portuguese but I think you use sobre or de for these purposes:

Eu penso dela ( I think about her)

Let's talk about something else.

He wrote a book about the history of America.

in Spanish if that helps at all:

sobre las tres= at about 3

sobre que= besides