function Change_Big_One(thumb){
document.getElementById('BigOne').src=thumb.src.replace("_th","")
}


//Place an <img> tag somewhere in the page and give it the id BigOne. This is the image that will be replaced.

//Put this in all the thumbnail <img> tags:

//onMouseOver="Change_Big_One(this)"

//If you use the same image both as a thumbnail and as the fullsize version, remove 

//.replace("_th","")