prueba los cambios que te envio resaltados:
<script LANGUAGE="JavaScript">
var myPix = new Array("../images/projects/7340_01.jpg","../images/projects/7340_02.jpg");
var thisPic = 0;
function showhide(max){
alert(thisPic+" "+max);
if (thisPic==max){nextimg.style.visibility="hidden";}
else {nextimg.style.visibility="visible";}
if (thisPic==0){previmg.style.visibility="hidden";}
else {previmg.style.visibility="visible";}
}function processPrevious() {
if (document.images && thisPic > 0) {
thisPic--;
document.myPicture.src=myPix[thisPic];
showhide(myPix.length-1);}
}
function processNext() {
if (document.images && thisPic < 1) {
thisPic++;
document.myPicture.src=myPix[thisPic];
showhide(myPix.length-1);}
}
</script>
y en el body:
<br><br>
<a href="javascript
:processNext();"
name="nextimg" id="nextimg" style="visibility:visible;">next</a><br>
<a href="javascript
:processPrevious();"
name="previmg" id="previmg" style="visibility:hidden;">prev</a><br>
<img src="" name="myPicture">
Saludos.