First, you must have the permission of the main site, otherwise
it is copyright violation.
Second, having downloaded the entire page, you need to write the
code that will parse it into usable chunks. I you know the
filename of the images you want you can just download them just
like you downloaded the page.
Usually writing the code to parse the page is not easy and then
you have to be prepared to rewrite it when the page changes.
-bill