If you want to dispose of them ask you doctor or your local hospital to see whether  they accept old x-ray films. If neither of them does, check your county’s website to see if they have a recycling program for such items or if a recycling center in your area does. Most old x-rays have silver in them, which means that they’re relatively valuable in the recycling market. There is usually a recycling center that accepts old x-ray films.

When getting new medical images done, ask the hospital/imaging center if they can provide them in digital format. most major hospitals and their related imaging satellite centers are switching to digital imaging, and many are no longer printing films unless requested.

CD's take up less space than a large envelope.

There are artists who may be interested in your x-rays. Many artists specialize in imagery of the body and x-rays provide a unique glimpse inside. Perhaps list them on a classified ad or material exchange site.

Some science teachers might be able to use them.

They are great to use as stencils for craft projects, and easy to cut with an exacto knife.