The image blocking is unrelated to the composing or sending system.
It still works that way if you send from Outlook to Outlook or to any other email client allowing to block images, web-base systems included.
Please keep in mind sending an email consisting of a single image will set spam scores sky high, often going beyond the threshold, either marking as spam or sending straight to junk folder.
I would suggesting sending that email to your account first (via flow, not send sample) and then inspecting the full headers for
X-Spam-Flag:
X-Spam-Score:
X-Spam-Threshold:
X-Spam-Exception