The thing to remember here is that the only way to hide (real world) data in an image is to reduce the amount of data in the picture... a blur or swirl leaves most if not all data just in the picture (although distorted) Any filter that removes data (such as pixelate or blacking out / whiting out) can be used to safely hide this data... Just remember to also strip out any unwanted meta data (Exif-data) and do not use layers but a 'flattened' version of the picture.