Large datasets such as Cultural Heritage collections require detailed annotations when digitised and made available online. Annotating dierent aspects of such collections requires a variety of knowledge and expertise which is not always possessed by the collection curators. Artwork annotation is an example of a knowledge intensive image annotation task, i.e. a task that demands annotators to have domain-specic knowledge in order to be successfully completed. Today, Lora Aroyo will present WebSci2014 conference the results of a study aimed at investigating the applicability of crowdsourcing techniques to knowledge intensive image annotation tasks. We observed a clear relationship between the annotation difficulty of an image, in terms of number of items to identify and annotate, and the performance of the recruited workers. Here you can see the poster and the slides of the presentation.