Mouse over buttons on .js file? Urgent-help

I wonder can anyone help please ...very very urgent! Am a web designer and am only just learning javascript and have just taken over some ones website to work on. The pages of this website come with an attached .js file for the 6 navigation menu buttons.

Somewhere in the .js script below is a code which means that whenever I mouse over the buttons on my web page navigation buttons, the button images disappear, leaving a blank space on the web page where the buttons should be. ( It may be something simple, but not understanding javascript well yet, I can't figure this out.)

Thought this might be a "rollover effect", with simple the rollover button images missing from my folder, as I just have 6 images and would need 12 if this was the case.

I have just 6 button images called :
dog_friendly_b1_over,gif
dog_friendly_b2_over.gif
dog_friendly_b3_over.gif etc

and no "rollover" images to replace them with. If the problem is that this is a rollover effect, and I need to make the rollover button images, not undertsanding the code, I wouldnt know what to call the 6 rollover buttons. But perhaps this is a diffrent problem altogether?

Probably a really dumb newbie in peril question, but please help if you can, I do learn quickly! Many thnaks anyone who helps.

Heres the .js file:

var clicked=""
var gtype=".gif"
var selstate="_over"
if (typeof(loc)=="undefined" || loc==""){
    var loc=""
    if (document.body&&document.body.innerHTML){
        var tt = document.body.innerHTML.toLowerCase();
        var last = tt.indexOf("dog_friendly2.js\"");
        if (last>0){
            var first = tt.lastIndexOf("\"", last);
            if (first>0 && first<last) loc = document.body.innerHTML.substr(first+1,last-first-1);
        }
    }
}

document.write("<table border=\"0\" cellspacing=\"0\" cellpadding=\"0\"><tr>");
tr(false);
writeButton("","index.html","dog_friendly_b1_over",100,18,"Home Page","",0);
writeButton("","contact.html","dog_friendly_b2_over",100,18,"Contact","",0);
writeButton("","catalog2.html","dog_friendly_b3_over",100,18,"View Attractions","",0);
writeButton("","maps.html","dog_friendly_b4_over",100,18,"Find Us","",0);
writeButton("","tariff.html","dog_friendly_b5_over",100,18,"Tarrifs","",0);
writeButton("","aboutus.html","dog_friendly_b6_over",100,18,"About Woodlands","",0);
tr(true);
document.write("</tr></table>")
loc="";

function tr(b){}

function turn_over(name) {
    if (document.images != null && clicked != name) {
        document[name].src = document[name+"_over"].src;
    }
}

function turn_off(name) {
    if (document.images != null && clicked != name) {
        document[name].src = document[name+"_off"].src;
    }
}

function reg(gname,name)
{
if (document.images)
    {
    document[name+"_off"] = new Image();
    document[name+"_off"].src = loc+gname+gtype;
    document[name+"_over"] = new Image();
    document[name+"_over"].src = loc+gname+"_over"+gtype;
    }
}

function evs(name){ return " onmouseover=\"turn_over('"+ name + "')\" onmouseout=\"turn_off('"+ name + "')\""}

function writeButton(urld, url, name, w, h, alt, target, hsp)
{
    gname = name;
    while(typeof(document[name])!="undefined") name += "x";
    reg(gname, name);
    tr(true);
    document.write("<td>");
    if (alt != "") alt = " alt=\"" + alt + "\"";
    if (target != "") target = " target=\"" + target + "\"";
    if (w > 0) w = " width=\""+w+"\""; else w = "";
    if (h > 0) h = " height=\""+h+"\""; else h = "";    
    if (url != "") url = " href=\"" + urld + url + "\"";

    document.write("<a " + url + evs(name) + target + ">");  

    if (hsp == -1) hsp =" align=\"right\"";
    else if (hsp > 0) hsp = " hspace=\""+hsp+"\"";
    else hsp = "";

    document.write("<img src=\""+loc+gname+gtype+"\" name=\"" + name + "\"" + w + h + alt + hsp + " border=\"0\" /></a></td>");
    tr(false);
}