function mostrarimagen(url_imagen,titulo,ancho,alto){
scr=0;
if (ancho>screen.width) {
ancho=screen.width-10;
scr=1;
}
if (alto>screen.height) {
alto=screen.height-60;
if (scr!=1) { ancho=ancho-(-16); }
scr=1;
}
win=window.open('','','width='+ancho+',height='+alto+',scrollbars='+scr+',resizable=no,toolbar=0');
win.moveTo(screen.width/2-ancho/2,screen.height/2-alto/2);
win.document.write ('\n');
win.document.write (''+titulo+'');
win.document.write ('\n');
win.document.write ('
\n');
win.document.write ('\n');
win.document.write ('\n');
}