A picture speaks a thousand words.
Now why is this relevant or even important? Well, unless you have been living in a cave for the last turn of the century, you will know that machines are taking over the planet. Even the internet is largely governed by Search Engines, which is mainly a system that follows a certain set of algorithms. A 'thousand words worth' image would simply be just one image to a machine.
So what would this mean to the typical blogger? Well, this is saying that unless you want your picture to be just like any other unnamed picture in the internet, you have to well, name it-- with words, so that a computer could understand. Search Engines like Google, Yahoo, or Bing rely on words to bring out their search results.
Instead of using underscore and space, use a hyphen in separating the words for the save image file name, seeing as spaces will come out as %20, while words with _ will be read as one long word.
Alt Text VS Title Text