Visual cryptography proposed by Naor and Shamir in 1995 encrypts a secret among several shares and decrypts the secret by observing the stacked shares using human visual system. This simple yet powerful mechanism for sharing images can be effectively applied on the mobile devices due to the popularity, portability and simple computation ability. When transferring the shares via local networks, especially among mobile devices using Bluetooth, a share with a smaller size is preferred in order to reduce the cost of transmission and the threat of being intercepted. Based on a random elementary block, we propose two simple algorithms to shorten the size of one share and examine the resultant degradation in the security. The tradeoff between them enriches the applications of visual cryptography in mobile devices.