#!/usr/bin/perl -w
use strict;

foreach (<DATA>) {
    s/^i ([^\n]+)$/makeImageTag($1)/e; #see Update
}

sub makeImageTag {
    my $foo = shift || 'no_image.png';
    print "$foo\n";
}

__DATA__
i howdy.gif
i foo.jpg
i bar.png
i blah.jpg