扩展包：

"bailangzhan/yii2-webuploader": "dev-master"

后端图片文件处理代码：

UploadForm.php 用于验证上传的文件

use yii\base\Model;
use yii\helpers\FileHelper;
use yii\web\UploadedFile;

/**
 * Class UploadForm
 * @package frontend\models
 * @property UploadedFile $imageFile 图片文件
 */
class UploadForm extends Model
{
    /**
     * @var UploadedFile
     */
    public $imageFile;
    public $savePath;

    public function rules()
    {
        return [
            [['imageFile'], 'file', 'skipOnEmpty' => false, 'extensions' => 'png,jpg,jpeg']
        ];
    }

    public function upload()
    {
        if ($this->validate()) {
            $relativePath = 'statics/upload/';
            $fileMd5 = md5_file($this->imageFile->tempName);
            !is_dir($relativePath) && FileHelper::createDirectory($relativePath);
            $this->savePath = $relativePath . $fileMd5 . '.' . $this->imageFile->extension;
            return $this->imageFile->saveAs($this->savePath);
        } else {
            return false;
        }
    }
}

控制器代码：

public function actionUpload()
{
    $model = new UploadForm();
    if (Yii::$app->request->isPost) {
        $model->imageFile = UploadedFile::getInstanceByName('file');
        if ($model->upload()) {
             return Json::htmlEncode([
                'code' => 0,
                'url'  => Yii::$app->params['domain'] . $model->savePath,
                'attachment' => $model->savePath,
            ]);
        }
        return Json::htmlEncode([
            'code' => 1,
            'msg' => '上传失败：' . $model->getErrors()['file'][0]
        ]);
    }
}