I'm sure you must have, Marilyn. It's awarded automatically when your proposal to move an image is approved. If you skip the proposal and just ask a moderator or admin to move it, however, you don't get the half-acorn.
You also get half an acorn when you move one of your own photos to a different entry and you get half an acorn when you import an image (your own or someone else's) from one of the forums to the database. If you do import an image, however, please check to make sure it isn't already in the database. The system recognizes duplicate images, but if the image has been cropped slightly or altered in any other way, the system won't recognize it.