An Imaging Guide to Mars
This guide will show you how to participate in the exploration of the Solar System. As Mars is the nearest Earth-like planet in many ways, we will start there. Mars is also a planet that has many probes around it and research going on into it, so you won't be short of materials.
The Planetary Data System 
The Planetary Data System: http://pds.jpl.nasa.gov/
Imaging Node: http://img.pds.nasa.gov/
The PDS is a system for storing all the images and data that come from space missions. It is organised into nodes and we are interested in the imaging node. By far the easiest way of using the system is by using the Planetary Image Atlas. There are other interfaces in the PDS and the websites of various missions. Mission teams may also release their own images separately but the data they are based upon always ends up in the PDS although this may sometimes take a few months. Check mission websites for the latest images.
Planetary Image Atlas: http://pds-imaging.jpl.nasa.gov/search/search.html
Raw Data 
A lot if images posted on space websites are in JPEG format which is an easy format for sharing images in. However, for investigative purposes it is not very good because the format uses what is known as "lossy" compression. Some of the original image is discarded in order to keep the size of the image file to a manageable size and the JPEG image may contain blocky and blurred sections as well as colour inaccuracies. Look for images in PNG or TIF format which are "non lossy". That way you will be able to see the image exactly as the probe took it. Some images may be in Planetary Data System formats like IMG. There are command line utilities to read them like ISIS which you may need for larger images but most of the time it's easier to use the graphics package GIMP and a plugin made specially to read these types of images.
See the GIMP plugin repository: http://registry.gimp.org/node/1627
Follow the instructions to install the plugin for your operating system and GIMP version: http://areo.info/gimp/
Note. The 2.4 version has been tested with version 2.6 of GIMP and works with it.
Basic Image Processing 
Once you have the IMG file loaded, or other non-lossy format, unless the image has already been processed you may need to use Colours --> Levels to balance the blacks and whites in the image to the point where features are visible to the eye. Notice the peaks and troughs in the GIMP levels dialogue. They correspond to details in the image and they will mark the boundary that you can drag the left and right arrow markers to. Don't drag over any parts of the graph if you can help it. As well as the left black and white marker on the right, the central arrow marker can also be dragged to change the balance of the image. Using the levels dialogue to properly process an image comes with experience. There is no single setting that will work with every image as every image is slightly different.
Some images need processing to remove vertical lines of noise.
Removing transmission errors 
Some image have transmission errors in them that can mess up the de-striping process. There is a method to remove these before de-striping.
WikiBook Editing Notes 
These resources may be relevant to editors ...
http://anserver1.eprsl.wustl.edu/ (MER Analyst Notebook - interface to MEr images and sites/sols)
http://www.highmars.org/niac/education/mer/mer00b.html (How to make MER colour images)
http://www.highmars.org/niac/education/mer/mer00e.html (explanation of confusing mars colours)