Prova questo, almeno potrai leggere i keycode rilevati...
codice:
<body onkeydown="k=event.keyCode; document.getElementById('pippo').value=k; if(k==39){location.href='gallery.php?ida=$ida&nf=$next&action=next' }; if(k==37){location.href='gallery.php?ida=$ida&nf=$back&action=back'}">  
<input type="text" id="pippo">