Astronomers take images with large professional telescopes because they want to use them to do some scientific research. They therefore need to be able to make measurements, such as the brightness of a star or the position of a planet.
This means that the images are not just pictures, but large amounts of data that can be used to reconstruct the image that the telescope was looking at during the observation. Essentially, the data lists the number of photons (packets of light) collected by every single pixel (minute region) on the detectors CCD detector - and that can run to many millions of pixels.
This data is usually stored on computers in special files and can be very large - a typical image data file from the Liverpool Telescope will have more than 4 million numbers in it! In order to deal with all these numbers and make the measurements that they need, astronomers use special computer programs called image analysis or processing software.