1. A non-transitory medium for storing data for access by an application program being executed on data processing apparatus, the medium comprising a data structure stored therein, the data structure comprising: a first image object stored in the medium, the first image object comprising a first set of related image data and first series data corresponding to a first series to which the first set of image data belongs; a second image object stored in the medium, the second image object comprising a second set of related image data and second series data corresponding to a second series to which the second set of image data belongs; at least one linking object persistently stored in the medium, the at least one linking object being an instance of a linking class which is configured to provide for instantiation of linking objects which each define, in use, a link from at least one source to a target, the at least one source corresponding to at least one of a series and a set of image data and the target corresponding solely to a series, in which the at least one linking object comprises: target data corresponding to a target series which is other than the first and second series to which the first and second sets of image data, respectively, belong; first source data corresponding to the first set of image data; and second source data corresponding to the second set of image data, the at least one linking object being configured such that when the at least one linking object is operated upon by the application program a first link is established from the first set of image data to the target data and a second link is established from the second set of image data to the target data, the at least one linking object further comprising a first source transformation entity which is operative when the linking object is operated upon by the application program to transform the first set of image data when the first set of image data is linked to the target data.