Photo doesn't seem to have attached to post:

There maybe a bracket you can buy to attach to the box and then hang the light on it.

There is a mounting hole in the center. All you need is a bracket that screws to the box and accepts a nipple in the center. The nipple goes through the hole in the center of the fixture and you screw a finial onto the stub of nipple sticking through the hole. You want to set the nipple length so that about 1/4" or so sticks through the hole so that, when the finial screws on, it draws the fixture up tight against the ceiling. You may have to fiddle around with the amount of nipple coming through the hole, but this is a standard way to mount such a fixture. Here's the bracket you want:

Here's what nipple looks like:

And here's a standard brass finial:

Should be able to get all that at a hardware store for $3.