Wall paintings go way back in the Basque Country, and one could say that the oldest example of it is the prehistorical cave art. In this Wikimedia Commons picture from the cave of Ekain, Gipuzkoa, you can see drawings of horses and steppe bisons:
Together with sixteen other caves (two of them Basque; Altxerri and Santimamiñe), Ekain forms part of the UNESCO world heritage Cave of Altamira and Paleolithic Cave Art of Northern Spain. Also in Iparralde, the French part of the Basque Country, there are several cave paintings, for example in Isturitz, Lower Navarre. These are some 15,000 years old, so the modern mural artist in the region are keeping a very old tradition alive.