(Disclosure: This user was me. Note that I didn’t say that quote markup must be used for licensing reasons, I said that quote markup makes clear that the image doesn’t fall under this license.)
There is no reason to give images special treatment over text.¹ If it’s not your content, but you’re allowed to include it, quote it.
Now, using the quote markup is not the only way to quote something. For inline quotes, using quotation marks is perfectly fine. And there might also be other ways to mark something as a quote (e.g., with a note). But there is no reason not to use quote markup for block quotes, and images are in most cases (like in OP’s example) included as blocks.
We have an easy way that works (just prepend a
>), why use something else?
Not a legal issue
To be clear, not to use quote markup is most likely not a legal issue for the author nor for Stack Exchange (as long as quoting the work is legally allowed, and the quote is marked in another sufficient way). But the quote markup helps people that want to make use of the content published on Stack Exchange sites:
What if the image is licensed?
Doesn’t change that it’s a good idea to quote markup:
If the image is licensed under an incompatible license, the use is only allowed thanks to quoting laws. The license doesn’t come into effect. Specifying the license can be useful, but quote markup should still be used.
If the image is licensed under a compatible license, and the use of the image isn’t covered by quoting laws, your use is covered by the license, and you might have to give attribution according to the license. Still, using quote markup helps people (for the reasons mentioned above).
Only if you edit the image (if allowed), quote markup must not be used. It’s no longer a quote, the image is now a derivative work (and attribution might be required).
> … (Markdown) generates
<blockquote>…</blockquote> (HTML). The
blockquote element is for quoted content, not just for text: "The
blockquote element represents content that is quoted from another source, […]".