Use https://github.com/elboletaire/Watimage as service, and after that use it from Volt (need cache solutions for resizes images, WatImage can do this if I'm not mistaken out of the box);
Resize images at time of uploading to all necessary sizes, it will decrease server's load;
Exoctic way, use Zephir to rewrite Watimage class and use as module for PHP;
what if image's width will be more or less than 100 but height will be 100 and vice versa?