codeigniter图像调整大小仅适用于$ config_img上的px

I'm trying to upload an image and everything is working fine, except the width and heigh of the resize. the strange part is, if I add px on the $config_img['width'] and $config_img['height'] I got an error saing:
"A non well formed numeric value encountered Filename: libraries/Image_lib.php"
What is obvious because the px on the value, but this way the image do resize.
Here is my upload image function:

function upload_foto_acompanhe_sua_obra($field) {
    $dir = realpath('assets/uploads/acompanhe_sua_obra');
    $config['upload_path'] = $dir;
    $config['allowed_types'] = 'gif|jpg|png';
    $config['encrypt_name'] = TRUE;
    $config['max_size'] = '500000';
    $config['max_width'] = '10024';
    $config['max_height'] = '7068';

    $this->load->library('upload', $config);
    $this->upload->initialize($config);

    $field_name = $field;

    if ($this->upload->do_upload($field_name)) {
        $dados = $this->upload->data();

        $this->load->library('image_lib');
        $this->image_lib->clear();

        $size = getimagesize($dados['full_path']);

        $config_img['image_library'] = 'GD2';
        $config_img['source_image'] = $dados['full_path'];
        $config_img['create_thumb'] = FALSE;
        $config_img['maintain_ratio'] = TRUE;
        $config_img['encrypt_name'] = TRUE;
        $config_img['width'] = '1092px';
        $config_img['height'] = '295px';


        $this->image_lib->initialize($config_img);
        $this->image_lib->resize();

        // Returns the photo name
        return $dados['file_name'];
    } else {
        $error = array('error' => $this->upload->display_errors());
        return $error;
    }

}

That's it, any help will be appreciated! Thanks!