PNG's IDAT chunk, which contains actual image data, is simply all lines of data compressed with the deflate algorithm. Once you inflate (uncompress) it, you hav

PNG's IDAT chunk, which contains actual image data, is simply all lines of data compressed with the deflate algorithm. Once you inflate (uncompress) it, you have all data and can compare if width and height match. This way it is quite easy to detect if additional data was added.