You can do it with the ImageMagick Perl bindings, or bindings into any other language of your choice:
#!/usr/bin/perl
use Image::Magick;
my($image, $x);
$image = Image::Magick->new;
$x = $image->Read('sars.png');
warn "$x" if "$x";
$x = $image->Resize(geometry=>'600x600');
warn "$x" if "$x";
$x = $image->Write('x.png');
warn "$x" if "$x";
$x = $image->Resize(geometry=>'400x400');
warn "$x" if "$x";
$x = $image->Write('y.png');
warn "$x" if "$x";
$x = $image->Resize(geometry=>'100x100');
warn "$x" if "$x";
$x = $image->Write('z.png');
warn "$x" if "$x";
The conjure command supports an XML-formatted Magick Scripting Language, but it's harder on my eyes than the Perl version, and the documentation on the Perl bindings is definitely better.