An image transmitting device for transmitting an image from an image source device to an image output device includes a second USB interface, an image receiver, a first memory, a wireless transmitter, a processor, and a switch. The image source device includes a first USB interface, and the image output device includes a wireless receiver. The second USB interface is connected to the first USB interface by a cable. The image receiver is used for receiving the image from the image source device. The first memory is used for storing the image received by the image receiver. The wireless transmitter is used for communicating with the wireless receiver and for transmitting the image from the first memory to the image output device. The switch is used for switching on or off the power voltage of the second USB interface and for driving the processor to switch on or off the image receiver.