Image Processing

There are many suitable image-processing programs. These instructions give the menu options for using the GIMP (Gnu Image Processor), which is of similar complexity as Photoshop. Other graphics applications offer similar features, but details of usage will differ.

You will probably want to use what you are familiar with, or these recommended less elaborate applications. You should be able to find the various tools for scaling, cropping, contrast, etc. in the menus.

Cropping and scaling

Digital cameras now take pictures with huge numbers of pixels, and the files are extremely large. Since the resolution of computer screens is typically about 75 pixels per inch, any picture of more than 800 x 600 pixels will be too large for a web page. To avoid wasting download time, crop away extra sides (see the lion, not mostly grass and sky) and scale the image to a size that appears suitable on your page (rather than specifying a smaller size in the IMG tag).

Jupiter's Great Red spot  This is a NASA picture of Jupiter's great red spot

For just the red spot,
  Select the rectangle tool, and mark out the rectangle you want to keep
  Use:  Image | Crop  -- and the rest goes away.
 

cropped red spot Scaling

Now, if we want it smaller, use Image | Scale image.
  Usually there is an option to preserve the ratio of height to width, so you don't compress or stretch just one dimension. With this setting, when you choose a different width (or height), the other dimension scales as well

Adjusting colours

For a background, there shouldn't be too large a range. Use brightness, etc. for getting a light (washed out) background for dark text, or a dark background for light text.
Tools | colour | Brightness-Contrast
light ret spot

light-red-spot.jpg

A brightened image of the great red spot
dark red spot

dark-red-spot.jpg

A darkened image of the great red spot

Picture too yellow, too blue? Color levels adjusted

Tools | colour tools | Levels | Auto
this automatically rebalances the amount of red, green, and blue. There are a lot of manual adjustments you can also play around with.
This transforms the great red spot thus: