How can I keep my storm door from dragging on the threshold?
Try tightening attachment of hinge to door and jamb.
If the attachment gets loose the door can sag.  It looks like you are missing a screw on the jamb side; center hole is empty.  I cannot imagine anyone would throw out a funky screw like this so maybe the escapee is still nearby in a drawer.
You should be able to tighten these with a hex wrench.  It might help to have an assistant lift the door slightly to oppose the sag and take the weight off the hinges when you tighten them.
Tighten all of them.  If there is a way to tell which hinge is the culprit I don't know it.
If this works that is great but it will happen again.  You might need to augment the holes in door and jamb to get a tighter purchase.