iOS or Android? On Android, you'll have to use a file manager to move the photos to a folder that PS Touch can monitor, like "Pictures." (File Manager HD is an excellent and free file management app available on Google Play. You can donate to the developer to get rid of the ads.)
ON EDIT: I should note: PS Touch does *not* modify your original image; it stays where it is, how it was. PS Touch only imports a copy to work with in a project.