¿Como poner una imagen que aparezca 5 seg. y se quite?

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alert("Pon aqui el mensaje que quieres!");
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<div id="postit">
<div align="right"><a href="javascript:closeit()" title="Cerrar"><font color="red"><b>X</b></font></a></div>
<b>Titulo:</b><br />
Aqui pon Mensaje que quieres mostrar
</div>
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