così al volo (non potendola provare) userei l'immagine trasparente:

es:
codice:
if (foto[i]){
document.write("<td height=\"190\"><a href=\""+cartella+foto[i] +".jpg\" rel=\"lightbox[k]\" title=\""+dida[i] +"\"><img src=\""+cartella+foto[i] +"k.jpg\" alt=\""+dida[i] +"\" width=\"200\" height=\"134\" hspace=\"5\" vspace=\"5\" border=\"0\" /></a>
<p style=\"text-align:center; margin:0\">"+dida[i] +"</p></td>");

}else{
document.write("<td height=\"190\"><a href=\""+cartella+fototrasparente +".jpg\" rel=\"lightbox[k]\" title=\""+dida[i] +"\"><img src=\""+cartella+foto[i] +"k.jpg\" alt=\""+dida[i] +"\" width=\"200\" height=\"134\" hspace=\"5\" vspace=\"5\" border=\"0\" /></a>
<p style=\"text-align:center; margin:0\">"+dida[i] +"</p></td>");

}
prova e fammi sapere.