What elements compound is used to produce x-ray images?

An ordinary x-ray machine such as in medicine or an airport scanner, consists of a cathode and an anode situated in an evacuated glass envelope.

Once accelerating voltages are over 50 000 Volts, x-rays will be generated at the anode.

Ordinary cathode materials suffice, and these commonly have a tungsten heater, maybe with a thorium coating, for this material emits electrons easily when heated.

The anode is the source of the x-rays, and commonly is angled such that the x-rays emitted will not travel straight back to the cathode. Because of the energies involved, the control of the direction of the x-rays is rather limited (read very difficult) and trying to focus the x-ray beam is indeed difficult.

Control of the beam is often accomplished by having a small tubular opening in an x-ray opaque material, thus simulating an attempt at collimation.

There are however x-ray sources that do not directly generate their own radiation, they instaed encapsulate a radioactive source which has inbuilt radiation properties. Cobalt 60 for example may be a source of neutrons, and one common use of this high energy source is for sterilizing medical equipment. Of course it is also used as a source for medical x-rays as well. In appearance, this material superficially looks like iron, and there has been at least one incident where a C60 source has been recycled (along with other scrap, in an horrific error) into reinforcing bars.

It may be that Cs137 is also used as an x-ray source, but my brief search failed to find a reference.