LOW / MODULES-121  Special case to solve the problem of palette element
duplication for shape with images. Our current macanims to retrieve
image resources instanciate a new FileResource each time it locates it,
thus for a same file, many FileResources are instanciated and so palette
elements are duplicated. This should be solve in 1.7, for instance by
creating a dedicated technology adapter for images. For now, just adress
the problem rapidly