## Read in all the images and numbers. There are probably 
## not that many, so don't worry about the efficiency of it too much.
@Images = &LoadImages();

## Figure out which page it is coming from. For this example, 
## let's say that each page (or even section of a page for 
## multiple banners on one page (yuck!))
my $pagenumber = &GrabNumber();

## Now just populate an hash of number strings. Since the page numbers 
## are just simply sequential in this example, we could 
## have used an array, but the hash lines up 
## neatly and allows us to easily change page "34" later 
## without counting lines, or worrying about order.

## Each pagenumber gets it's own grouping of images.
my %imagegroup = (
1  => "1,2,5,7",
2  => "2,4",
3  => "*",
4  => "1,2,3",
5  => "4,8,10",
);

## For the above, page "1" gets images one, two, five, and seven, etc...
## Note how pagenumber "3" is always going to get all images.

## Then just generate a random image. Again, because the 
## list of images will probable be small, we don't worry 
## too much about efficiency here (besides, it's early!)

## Default to "all images"
my $grouping = $imagegroup{$pagenumber} || "*";
my $bail=0;
{
  my $guess = int rand @Images;
  last if $grouping eq "*" or $bail++ > 1000;
  $grouping =~ /\b$guess\b/ or redo;
}

## Now $Image[$guess] is the one to display...