The popular saying "A picture is worth a thousand words" is, well, technically not true. Use this picture for an example. You see the man on his knees with anguish on his face and fear in his eyes. IT looks like he is saying no, but we wouldn't know without the caption. What about the man with the knife? WHO is he? WHERE is he? WHAT is going on? WHEN was this picture taken.
Research has shown that pictures and graphics attracted the most attention and were looked at first. Then headlines and advertising and then text. Now you know why captions are so important.
Captions can be one word or a mini-article if it is not accompanied by a article. Check your facts, this is journalism and if you get the names etc. wrong, you are in big big trouble. Quotes can be used as a caption but please do not use humor unless it is definitely warranted.