import bpy

for obj in bpy.data.objects:
    if obj.active_material is not None:
        if obj.active_material.name.startswith('textures'):
            print(obj.active_material.name)
            mat = obj.active_material
            tex = 0
            if mat.texture_slots[0] is not None:
                continue
            try:
                tex = bpy.data.textures[mat.name]
            except KeyError:
                tex = bpy.data.textures.new(mat.name, 'IMAGE')
                try:
                    tex.image = bpy.data.images.load('path/to/assets/' + mat.name + '.jpg')
                except RuntimeError:
                    print(mat.name + ' not found')
                    continue
            slot = mat.texture_slots.add()
            slot.texture = tex