subjectAltName is for DNS names, IPs, emails, etc. Use a custom extension (as shown) or the certificatePolicies extension with a UserNotice containing the image (more complex).

Suggested follow-up reporting angles (if turning this into a published feature)

Let us address why people search for this incorrectly:

Users may confuse PFX with image-heavy formats like PDF or assume it is a general-purpose "container" for sensitive documents. Risks of "Free Online" Tools