The plague is the punishment that comes from Apollo. In the beginning of the play, Kreon arrives and reports what he has learned from going to Delphi: Apollo orders the Thebians to take revenge upon whoever killed King Laios. The symbolism of the plague is that it represents the "disease" of regicide and patricide, as well as the incestual relationships which develop after the death of King Laios as Oedipus marries his mother unknowingly.
When Kreon returns from Delphi, he tells Oedipus,
The god commands us to expel from the land of Thebes
An old defilement we are sheltering
It is a deathly thing, beyond cure:
We must not let it feed upon us longer. (99-112)
Oedipus asks Kreon how Thebes can be rid of this defilement. Kreon replies that the riddance can be effected by exile or by death as the only ways--"blood for blood." Further, he relays to Oedipus that Apollo commands the they take revenge on the murderer. Here the tragic irony of the drama begins.
Regarding the term picturesque, it is best understood in its relationship to the aesthetic ideals of the beautiful, something that is in balance and harmony with nature, and the sublime, the quality of perfection that evokes awe and great reverence. To consider the plague as picturesque seems a distortion of the term picturesque since the plague is hideous and frightening as it is spreading through Thebes.