# Elsewhere in the code, we get an img_id, do a SELECT to find what
# album_id it belongs to, then use the SELECT below to get all of
# the img_id's that belong to that album. We use flattenArray() to 
# turn the arrayref from DBI into a plain old array.
my @image_idx = flattenArray( @{$dbh->selectall_arrayref("SELECT img_id FROM images WHERE album_id = $image_id")});

# Find what place our target img_id is in the array.
my $idx_loc = indexArray($img_id, @image_idx);

# Get img_id's from array that come before and after the target.
my $prev_img = $image_idx[$idx_loc - 1];
my $next_img = $image_idx[$idx_loc + 1];

# Thanks to merlyn, tilly and particle
# http://perlmonks.org/index.pl?node_id=151120
sub flattenArray {
  my @flattened; # Will be in reverse order
  while (@_) {
    my $last = pop;
    if (UNIVERSAL::isa($last, "ARRAY")) { push @_, @$last; }
    else { push @flattened, $last; }
  }
  return reverse @flattened;
}

# My apologies, but I took this from a golf thread on PM. I've lost
# the node number and apologize to the author. Please let me know
# so I can credit you for this.
sub indexArray(@) {        
    my $s=shift;
    $_ eq $s && return @_ while $_=pop;
    -1;
}