AI-generated images and AI-generated text both have their "tells." Just because an author chooses to use an AI-generated image doesn't mean they didn't write the post.
By the very nature of ai-generated content, a typical internet user is going to be exposed to a huge mass of ai posts. Then, to associate ai images with ai text is a logical step.
Is it? People tend to have a much easier time creating prose than they do art. Look at us here, right now!