I don't like to split hairs. But in general we understand an advertisement to be a public notice in e.g. a newspaper, that is paid for by a company and wants to sell you something. It's a marketing tool. And if it's a series, we can call it a 'campaign'.
I wouldn't call any message on a billboard an advertisement. And I assume, few people would.
I wouldn't even like to call what is put here onto the billboards a 'message'.
But let's approach it from another angel: Do you see any difference between advertisement and art?