Tabl10s, you can't do it directly, you have to finesse the system. After submitting your post, edit it using "edit my post" and add the standard html tags pointing to an image stored elsewhere on the web. Not difficult, but you have to act while you can still edit, and the edit feature does not work on some threads (e.g., multiple page threads and some actively moderated threads).
Example:
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Example:
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