A glass doorknob graced the old door.
It should have a period and it is a declaritive sentence because it declares something.An interrogative sentence asks a question. An exclamatory sentence is a more forceful version of a declarative sentence. Say these out loud:
"A glass doorknob graced the old door!"
Doesn't that sound weird and too excitable?
"A glass doorknob graced the old door?"
That doesn't even make ANY sense!!
Therefore, it is a period declarative.
I love your pic, I love horses, and I hope this helps!!! (: