Wordpress doesn't apply rotation to iPhone images
Many cameras store the orientation (portrait or landscape) in the EXIF metadata of the image file. The actual pixel data is not rotated, but the metadata contains the information necessary for the viewing program to display it correctly.
Wordpress seems to apply the rotation to resized versions, but ignore it for the full size images. There is a plugin that takes care of this problem: Image Rotation Repair (I haven't used this myself, though).
Note: This will only apply to images that are uploaded to and processed by Wordpress, not to images linked from a different location via URL.