How do you caption an uploaded image?

Anònim fa 10 anys updated by Alan Schaaf (Founder) fa 10 anys 3
I did it before but i can't do it again. Please help as i love this site!
I would like to know how to comment on other peoples images too.
i believe only popular images that becomes part of the gallery have commenting feature right now. 
And how would you then delete said caption/comment?