dont forget to backup your files before editing it!
1 files to modify
-overall_header.html
im using the prosilver template, but this work with any forum style.
Step #1
first locate the overall_header.html in root/styles/prosilver/template
open it with notepad++
Insert the following javascript inside the <head></head> tag:
<script type="text/javascript" language="javascript">
function is_loaded() { //DOM
if (document.getElementById){
document.getElementById('preloader').style.visibility='hidden';
}else{
if (document.layers){ //NS4
document.preloader.visibility = 'hidden';
}
else { //IE4
document.all.preloader.style.visibility = 'hidden';
}
}
}
//-->
</script>
Step #2
Now replace the <body id="phpbb" class="section-{SCRIPT_NAME} {S_CONTENT_DIRECTION}">
by: <body onload="is_loaded();" id="phpbb" class="section-{SCRIPT_NAME} {S_CONTENT_DIRECTION}">
Step #3
insert this html code just under the <body onload="is_loaded();" id="phpbb" class="section-{SCRIPT_NAME} {S_CONTENT_DIRECTION}">tag:
<div id="preloader" style="position:absolute; left:30%; top:290px; width:380px; height:120px;">
<center>
<table border="0" align="center" cellpadding="6" cellspacing="0" style="font-family:Arial, Verdana; border: 2px solid #006633;">
<tr>
<td style="text-align:center; background-color:#ffffff">
<font style="font-size:120px; font-weight:bold; color:#0099ff">!</font>
</td>
<td style="text-align:center; background-color:#0099ff">
<font style="font-size:28px; color:#ffffff; text-align:center;">Loading ... please wait</font>
</td>
</tr>
</table>
</center>
</div>
costumize it to your language by changing: "Loading ... please wait" for the language you want.
enjoy!

exemple: Click here