In your view you only pass an id into your DeleteFiles function which is why photoFilename is null:
@Html.ActionLink("Delete", "DeleteFiles", "Account", new { id = @item.PhotoID }, null)
Two suggested options:
1) Add the ImagePath to the parameters you're passing:
@Html.ActionLink("Delete", "DeleteFiles", "Account", new { id = @item.PhotoID, photoFilename=@item.ImagePath }, null)
2) Locate the database item in your DeleteFiles first via its ID and then extract the path from that. That way you can drop the photoFilename parameter entirely.