A particular user is spamming one of my questions about Albus Dumbledore with an image of him wearing 'socks'.
How do I report him?
You can report a user for spam by flagging the spam comment/post. In this case, you did so on the first comment, which I deleted. If the user continues to spam, I'd use a custom flag on the spam comment/post with a complaint that the user is continuing to spam. This will signal to the moderators that we need to investigate for other instances of spam. The more information you can give us, the better.
You can also use meta as you've done here. After seeing this meta post, another moderator deleted the second comment. Let us know if this behavior continues.