Hi,
I let users upload images to my website, but sometimes they upload huge resolution images.
So I want to resize these images after upload.
Just for beggining I use this code:

$original = File::get()->byID($file->ID);
$file720 = $original->SetRatioSize(720,720);
$file720->write();	

No help?

You could extend onAfterUpload() and quietly replace the original with the resized one. Something like:

class NotTestedResizeImageOnAfterUploadExtension  extends DataExtension {
	public function onAfterUpload() {
		$imgObj = $this->owner;
		$resizedImg = $imgObj->SetRatioSize(720,720);
		file_put_contents($imgObj->getFullPath(), file_get_contents($resizedImg->getFullPath());
	}
}

Great idea, will gonna try it :) Thanks!