From their README:

    <|im_start|>user
    What is this sign about?\n<image>
    ###
    <|im_start|>assistant
    The sign is about bullying, and it is placed on a black background with a red background.
    ###

Any idea how you fill out that <image> bit?

No, but the GitHub repo linked from there includes a Gradio Demo UI which has an image field, so it should be possible to reverse engineer.