Well I have not written a hub for a while, however in the help section you can find a lot of good tips on this subject.
I find pictures compliments the story/hub/poem.
I always add pictures that tell the story of my post, although the picture may seem to attract the attention as opposed to the main topic, it also attracts more readers.
I myself like to read a book more so if it is double spaced and adds beautiful pictures, otherwise the post/story becomes too crowded and overwhelming. This is just my opinion and how I feel. Sometimes a picture tells the whole story.
Of course I do give credit to the author or where the picture came from (like google).