Plain rotation, scaling, and/or translation of an image can lead to loss of watermark synchronization and thus authentication failure. The block based approaches in particular are sensitive to geometric distortions due to the need for repositioning the block's rectangular grid. In this paper, we propose a novel block based approach for watermarking image objects in a way that is invariant to RST distortions. With the term "image object" we refer to semantically contiguous parts of images that have a specific contour boundary. The proposed approach is based on shape information since the watermark is embedded in image blocks, the location and orientation of which is defined by Eulerian tours that are appropriately arranged in layers, around the object's skeleton. The skeleton of the object (MAT transform) is defined by the shape of its boundary and not only is invariant to RST transformations but is also robust to cropping, clipping and other common deformation attacks. Results on the Caltech database are reported, showing the advantages of the proposed scheme.