The proportions of your original photo are not 6x4, and the printer is resizing the photos to give you the size print you're asking for.

There is precious little you can do to keep all the photo AND get the size you want. However, if you go to Edit in iPhoto, choose Crop, and set the Constrain Proportions to 4x6, then you can gain control over HOW the photo gets cropped by moving the box around to crop only the least important areas.

Hope this helps.

- Joe U.