As with anything else, images come in different kinds and serve
different purposes. Sometimes, a small size is important (for web sites)
and at other times, retaining a high colour depth in all its glory (a
family portrait) is what you want. GIMP can handle all of this, and
more, primarily by converting between three fundamental modes, as seen
in this menu. In order to switch your image to one of these modes, you
open it and follow that menu and click the mode you want.
Dialog for changing the mode
- This is the default mode, used for high quality rich colour images.
This is also the mode to do most of your image work with including
scaling, cropping and even flipping as it gives the computer more
information to work with. This extra information also makes RGB Mode the
largest to store as a result.
A little bit of detail if you are interested. Each pixel or point when
in this mode consists of three different components. R->Red, G->Green,
B->Blue. Each of these in turn can have an intensity value of 0-255. So,
at every pixel, what you see is an additive combination of these three
components. All these combinations result in a way to represent millions
- This is the mode usually used when file size is of concern, or when
you are working with images with few colours. It involves using a fixed
number of colours, 256 or less, at each point to represent the colour at
that point. The defaults to attempting to figure out an "optimum
palette" to best represent your image. Try it, you can undo it if you
don't like the results, or use a custom palette or more colours.
Dialog “Change to Indexed Colors”
As you might expect, since the information needed to represent the
colour at each pixel is less, the file size is a lot smaller. However,
sometimes, there will be options in the various menus that seem to have
been "greyed" out for no apparent reason. This usually means the filter
or option cannot be applied when your image is in its current mode.
Changing the mode to RGB as outlined above should solve this issue. If
that doesn't work either, perhaps the option you're trying requires your
layer to have the ability to be transparent. This can be done just as
easily via (Image)->Layer->Transparency->Add Alpha Channel.
Add Alpha Channel
- In case you want to convert your brilliant colour image to something
that's black and white (with a lot of shades of grey), this is one of
the easiest ways in which to do it. Some photos do look a lot fancier
when displayed in grayscale. Again, if you're interested in some detail,
this is achieved by taking the RGB values at the pixels in your image,
and suitably weighted averaging them to get an intensity at that point.
There is no need to convert an image to a specific mode before saving it
in your favourite format, as GIMP is smart enough to export.