my $pre="image";
my $post="jpg";
my @files=<$pre.*.jpg>;

@files=reverse sort @files;
#should work because you don't have image.10.jpg
#if you do have image.10.jpg, then you will need to use
#image.09.jpg for example

foreach (@files)
{
	next unless m/^$pre\.(\d)\.$post$/; #just to be sure
	rename_and_log($_, "$pre." . ($1 >= 8 ? 9 : $1+1) . ".$post\n");
}