function swap(sImgName,sLyrName){
        var sDocImgPath = document.images[sImgName];
        var sImgSrc = sDocImgPath.src;
        var iDotPos = sImgSrc.lastIndexOf('.');
        var sExt = sImgSrc.substring(iDotPos,sImgSrc.length);
        var sPath = sImgSrc.substring(0,iDotPos-1);
        if(sImgSrc.charAt(iDotPos-1) == 'p')
                var sSwapSrc=sPath+'a'+sExt;
        else if (sImgSrc.charAt(iDotPos-1) == 'a')
                var sSwapSrc=sPath+'p'+sExt;
        sDocImgPath.src = sSwapSrc;

}