How does a sattelite send a signal?

Plain and simple radio. The images are taken with a digital camera, that translates them into digital information (a sequence of numbers) and that is radioed back. The technology is no different from any digital camera you might have or transmitting .jpg images.

The only difference is that from so far away, the radio signals are very weak. But as long as we can detect them, we can read them and translate them back into images. NASA uses radio telescopes for this purpose as they are designed to detect weak signals.

The satellites and especially remote probes have parabolic dish antennas for sending and receiving radio signals. The parabolic dish acts a lot like the reflector in a torch does and forms a parallel beam which is aimed towards earth. On earth there are huge parabolic antennas which focus and receive the very faint signals, and send commands back

By radio. The ones nearby in Earth orbit just use a normal antenna. Interplanetary space craft have large dish antennae to concentrate the radio energy into the narrowest possible beam aimed at Earth while special ground stations use even bigger dish antennae and ultra-sensitive receiver electronics located in remote places far from Earthly radio interference to detect the weak signals.

Radio signals.

using a radio, run by batteries. MAYBE those batteries are charged by Solar Power, IF the satellite is close enough to the sun.

But it take a RALLY LONG TIME to send pictures, often 24 - 48 hours per picture

Through radio. The imagers on board store the information digitally, then it's transmitted via radio back to Earth.

Radio.

I believe the information is sent via radio waves because they can travel the farthest and through the Earth's atmosphere easiest