We present an application of digital image processing to the analysis of the conservation of a medieval mosaic. The art piece is the "The Last Judgement" mosaic, situated on the wall of St Vitus Cathedral in Prague, Czech Republic. A 19^th century historical photograph of the mosaic was compared with a photograph of its current reconstructed state. The images were first 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 between the current and historical photographs were identified.