by: SSK
The meaning is, basically, that even new things, such as Harry Potter, can work themselves into great influential and recognized works of art (art being either painting, such as The Scream, or literature, such as Harry Potter). So, not only is the man screaming because he's just seen "the excaped convict", but also because he may be replaced by him.
And, yes, that is Sirius walking away in the background.
I had fun making this - thank you Photo-Impression!