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The Image archive model described here has been developed within the scope of the IDHA (Images Distribuées Hétérogènes pour l’Astronomie) project. The IDHA project led by CDS, associates image processing scientists of LSIIT (Strasbourg), and astronomers of several French Institutes including TERAPIX from IAP, and Queen’s University (Belfast). IDHA goal is to describe the information content of image archives and datasets. The metadata are designed as objects interacting with each other, using the standard UML notation. To describe the user logical approach to such image archives we studied different scenarii for the queries of images and their scientific interpretation. We emphasize the need for scalability in the metadata, from description of one single astronomical image to a complete information about an image collection such as a survey or a telescope archive.