use 5.010;
use strict;
use warnings;
my $text = do { local $/ = <DATA> };

my ($start, $end) = map quotemeta, qw([img] [/img]);
$text =~ s{($start(.+?)$end)}{
	my $link = $1;
	my $uri  = $2;
	$uri =~ /\.(?:jpe?g|png|svg|gif|bmp)$/ ? $link : 'Invalid'
}eg;

print $text;

__DATA__
Foo
[img]http://example.com/[/img]
Bar
[img]http://example.com/logo.jpeg[/img]