In 1914, the French Army still retained twelve regiments of cuirassiers. They still wore the cuirass (with a cloth cover) and plumed helmet on active service during the first weeks of the war. This reflected the historic prestige of this branch of the cavalry, dating back through the Franco-Prussian War to the campaigns of Napoleon. Upon mobilisation in 1914 the only concession made to active service was the addition of a cover of brown or blue cloth over the shining steel and brass to make the wearer less visible. Within a few weeks, most French regiments stopped wearing the cuirass, as it served no real purpose in this new war. It was not however formally withdrawn until October 1915. (source: Wikipedia).
That’s the reason why this picture is rather uncommon: this French cuirassier wears a bleu horizon uniform with his armour plate and helmet without protecting cloth.