Это должно работать:
var result = db.PhotoAlbums.Select(albums => new PhotoAlbumDisplay
{
AlbumID = albums.AlbumID,
Title = albums.Title,
Date = albums.Date,
PhotoID = albums.Photos.Select(photo => photo.PhotoID).FirstOrDefault().ToString()
})
.OrderByDescending(item => item.AlbumID);
В синтаксисе запроса:
var result = from albums in db.PhotoAlbums
orderby albums.AlbumID descending
select new PhotoAlbumDisplay
{
AlbumID = albums.AlbumID,
Title = albums.Title,
Date = albums.Date,
PhotoID = albums.Photos.Select(photo => photo.PhotoID).FirstOrDefault().ToString()
};