iOS: Understanding photo sorting
The order in which photos synced from your computer are sorted in the photo albums of your iPhone, iPad, or iPod touch depends on the software you are using on your computer. Learn how this works in iPhoto, Aperture, and other software.
By default, photos sort according to the date modified (in ascending order).
When syncing photos from a Mac or PC, some apps may sort your photos in the order you specify in the app (such as iPhoto or Aperture). If your app does not support this feature, photos will be sorted by the date modified.
In addition to albums and events, synced photos appear in a unified category called Photo Library (on iPad, this is the Photos tab). Photos are not duplicated. This category will also sort photos in ascending order by the date modified.
If you are using an earlier version of iOS
iOS 4 sorts photos by the Date Taken tag of the photos. iOS 3.1.3 and earlier sort photos alphanumerically by the photos' file names.