We present an application of digital image processing for the analysis of the medieval mosaic conservation. The reconstructed art piece was the ”The Last Judgement” mosaic, situated on the wall of the St. Vitus cathedral in Prague, Czech Republic. The historical photograph of the mosaic from the ½ Ø century was compared with the photograph of the current state. The images were firstly preprocessed to increase their quality (noise reduction, deblurring). In the second stage, geometrical differences between images were removed by means of image registration techniques. Finally, differences of the current and historical photographs were identified.