On the example of digital images there are investigated properties of the discrete Fourier transform (DFT) used to embed information into the phase spectrum. The investigation helped to form a new steganografic algorithm that can be used in case of non- compressed images. Peculiarity of the algorithm is changeable size of information put into the blocks of stego-image. Characteristics of the suggested algorithm are comparable with the analog ones, they allow to get the correct information without mistakes.