To resize image in magento use below code and pass image url as parameter in Varien_Image object.

/** Varien Image object for image url */

    $imageObj = new Varien_Image($imageUrl);

/** Set constrain */

    $imageObj->constrainOnly(true);

/** Keep Image Aspect ratio */

    $imageObj->keepAspectRatio(false);

/** Keep Image frame */

    $imageObj->keepFrame(false);

/** Keep Image Transparency */

    $imageObj->keepTransparency(true);

/** Set image background Color */

    $imageObj->setImageBackgroundColor(false);

/** Set background color */

    $imageObj->backgroundColor(false);

/** Set image quality */

    $imageObj->quality(100);

/** Set watermark Image Opacity */

    $imageObj->setWatermarkImageOpacity(0);

/** Set image size and */

    $imageObj->resize(182, 126);

/** Save image at given new file path */

    $imageObj->save($imageResized);