We look where other people are looking. We follow lines of sight, gaze, arrows, or even long lines. This short resource on visual grammar calls such things, “vectors” (last two pages) and reminds us that they are powerful ways to focus attention.
I use this technique all the time on my blogs. Here’s a picture of a recent post:
The image of the fish reinforces the location of the main text. The fish is “pointing” to where you want your reader to look. Compare it to this:
Not as good. Now your attention is being drawn away from the main text into less relevant marginalia. It would be even worse if there weren’t a sidebar on the right.
What if you have the perfect picture, but pointing the wrong way? Any decent graphics editor will be able to flip the picture into a mirror image.
And the moral of the story is:
If you have pictures of people or animals or arrows on your poster, make sure that their gaze or the line of attention is directed inwards, towards the middle of the poster if possible.
Of course, this can sometimes run afoul of other conventions. I once was told that in anatomical drawings the tradition is always to have anterior end of the animal facing left, for example.