This might not be a possibility as it would require tinkering with the forum software, but I'll mention it anyhow...
Would it be possible to eliminate the "post" button before a person previews their message? This might be a solution to the chronic problem of people not copying and pasting their Imageshack links into their posts. If they can see that the image does not show up in the preview, it might make them realize the problem. Conversely, it might make the problem worse as people become impatient about having to hit two buttons instead of one.
