QUESTION:
In response to your question:
What is a methane image and what does it tell us?

ANSWER from Reta Beebe on May 30, 1996:
When you are observing with Hubble, it's a black and white
camera, there's no color detected by the camera itself. This camera
is capable of seeing light much farther in the UV (ultra-violet
segment of the electromagnetic spectrum) than we are used to
thinking about, and farther into the infrared that the human eye can           
see. In front of the camera there is a filter wheel device. You can
select the filter you place in front of the camera. That determines the
color of light that is coming through to your camera. 

In one orbit of the spacecraft, since Jupiter is such a bright object, we 
can get about ten images. So it's possible to select a series of colors. You
could look in the UV, the near UV, blue , green, red, and use your
blue, green and red to make your color image... Then you can
move on out to the near-infrared where methane gas absorbs very
strongly in some colors, and there are a pair of filters aboard
Hubble that allow you to take one image where the methane gas is
absorbing so strongly that the photons of light that go in have little
chance of coming back, so anything that appears bright in that
image are high clouds, or hazes, that reflect the light before it goes
down and is absorbed by the methane. The other filter is so
designed that it only allows a band of light to pass in a region that
methane gas does not absorb, so the light goes down and reflects
off of the ammonia cloud deck. And when you compare those two
you have a very good measure of which clouds are high and which
clouds are lower in the atmosphere... (the) vertical structure, which
you can then compare to what you see in the earth's atmosphere.                
...It's quite possible that we could schedule the observation so that
we could have (Jupiter's colorful volcanic moon) Io crossing in
front of Jupiter, and then we would observe Io in a series of
colors. Now Io is so bright itself that -- it reflects so much of the
sunlight -- that if we were attempting to take a methane picture of
Jupiter, Io would be over-exposed, it would be just a white blotch.
But it does not damage the cameras, so this would not be a major
problem.