This paper presents GAT, a Graphical Annotation Tool for still images that works both at the global and local scales. This interface has been designed to assist users in the an- notation of images with relation to the semantic classes de- scribed in an ontology. Positive, negative and neutral labels can be assigned to both the whole images or parts of them. The user interface is capable of exploiting segmentation data to assist in the selection of objects. Moreover, the annota- tion capabilities are complemented with additional function- alities that allow the creation and evaluation of an image classi er. The implemented Java source code is published under a free software license.