If the sentence given by the storyteller describes the image too precisely, all the players will easily find it and so he won't score any points.
On the other hand, if his sentence has very little to do with the image, it's quite likely that nobody will vote for him, and again he will score no points.
Balance issues with inside jokes.
Play the game often. You are not a master tactician your first game of chess, and so Dixit players will not be the world's most creative-yet-decipherable Storyteller in the first game.
Try to apply previous clues to cards in your hand in new ways. Reusing a clue in a completely new way is an awesome way to make the other players scratch their heads, especially if everyone knows you'll be twisting it.
So the challenge for the storyteller is to find a sentence that is neither too descriptive, nor too abstract, so that there is a chance that only some players find it, but not all.
At the beginning, this can be fairly hard to achieve, but you'll see that the inspiration comes more easily after a few turns.