While public image-sharing platforms enable users to share images easily, security of published images becomes more important. An effective method to protect a published image is encrypting it before its upload. However, the encrypted result has to be viewed as a correct digital data by the sharing platform. Normally, the digital data should be an image (and often a JPEG image). Therefore, an encryption method which can preserve the image format after encryption is needed. And the main objective of decryption is to reconstruct an image while preserving image quality. In this paper, we propose three JPEG image encryption schemes and compare them. All these three schemes use the same encryption algorithm but are integrated into different steps of the JPEG compression process. The first one encrypts image before DCT in spatial domain. The second one encrypts image after DCT in frequency domain. The third one encrypts image after quantization. These three schemes undergo JPEG processing and obtain JPEG image as the encrypted result. Further, we get the performances of these three schemes through comparing the runtime, Peak Signal to Noise Ratio (PSNR) and Universal Image Quality Index (UIQI), and conclude that which one is closest to our needs.