perche' scrivere
var frames = new Array(url_images.lenght);

e' come scrivere
var frames = new Array(4);

cioe' crei un array di 1 elemento... di valore 4

NOTE:

If you specify 'language="Javascript1.2"' in the <SCRIPT> tag and use a single numeric parameter with the Array constructor, it will be seen as the value of a single element of the array rather than the number of elements you want that array to contain.
tratto da: http://www.devguru.com/Technologies/...ref/array.html

ciao