şunlara bir bakın isterseniz;

http://stackoverflow.com/questions/13596794/resize-images-with-php-support-png-jpg

http://php.net/manual/tr/function.imagecopyresampled.php

http://php.net/manual/tr/function.imagecopyresized.php

https://www.developphp.com/video/PHP/Image-Resize-Function-Tutorial-jpg-gif-png-Change-Size

ek olarak
http://www.trkodlama.com/makaleler/php-ile-resmi-yeniden-boyutlandirma-471.html