Hola, Gaby
Intenta usar innerHTML:
<script type="text/javascript">
function susti(p)
{
if (p==1) document.getElementById("imag").innerHTML = 'texto'
else document.getElementById("imag").innerHTML = '<img src="algo.gif">'
}
</script>
<span id="imag" onMouseOver="susti(1)" onMouseOut="susti(0)"><img src="algo.gif"></span>