var the_imageoff = new Image();
var the_imageon = new Image();

// for JPGs

function img_actJ(imgName){ 

if (document.images){ 
the_imageoff[imgName] = new Image();
the_imageon[imgName] = new Image();
the_imageoff[imgName].src = "images/" + imgName + ".jpg"
the_imageon[imgName].src = "images/" + imgName + "_ON.jpg";

document.images[imgName].src = the_imageon[imgName].src;
}
}

function img_inactJ(imgName){ 

if (document.images){ 
the_imageoff[imgName] = new Image();
the_imageon[imgName] = new Image();
the_imageoff[imgName].src = "images/" + imgName + ".jpg"
the_imageon[imgName].src = "images/" + imgName + "_ON.jpg";

document.images[imgName].src = the_imageoff[imgName].src;
}
}

// for GIFs

function img_actG(imgName){ 

if (document.images){ 
the_imageoff[imgName] = new Image();
the_imageon[imgName] = new Image();
the_imageoff[imgName].src = "images/" + imgName + ".gif"
the_imageon[imgName].src = "images/" + imgName + "_ON.gif";

document.images[imgName].src = the_imageon[imgName].src;
}
}

function img_inactG(imgName){ 

if (document.images){ 
the_imageoff[imgName] = new Image();
the_imageon[imgName] = new Image();
the_imageoff[imgName].src = "images/" + imgName + ".gif"
the_imageon[imgName].src = "images/" + imgName + "_ON.gif";

document.images[imgName].src = the_imageoff[imgName].src;
}
}