The graphics tools chapter explains some image programs that can be called from the command-line. While I have found image programs that can be used from the command-line, zgv is the only one I've ever heard of, I did not find them very useful. All the tools listed use the X windowing system to work and simply run from the command line (so they can be scripted/automated if necessary).
montage r34.jpg r32.jpg skylines* skyline_images.miff
Note: Note that the images are converted to the same size (scaled) so they can be tiled together.
To convert the file format of an image to another image format. convert is used to change a files format, for example from a jpeg to a bitmap or one of many other formats. convert can also manipulate the images as well (see the man page or the ImageMagick site).
convert JPEG: thisfile.jpg PNG: thisfile.png
display is used to display (output) images on the screen. Once open you are can also perform editing functions and are able to read/write images. It has various interesting options such as the ability to display images as a slide show and the ability to capture screenshots of a single window on-screen.
Command syntax (for displaying an image):
Be careful when opening multiple large sized images (especially on a slow machine) and putting the slide show on a small delay between image changes. Your processor will be overloaded and it will take a significant amount of time to be able to close ImageMagick.
mogrify is another ImageMagick command which is used to transform images in a number of different ways, including scaling, rotation and various other effects. This command can work on a single file or in batch.
For this you could type:
showrgb is used to uncompile an rgb colour-name database. The default is the one that X was built with. This database can be used to find the correct colour combination for a particular colour (well it can be used as a rough guide anyway).
Please note:: All tools listed, excluding showrgb are part of the ImageMagick package. Type man ImageMagick for a full list of available commands. Or see the ImageMagick site ImageMagick for further information.
This example has come been used from (unedited) “CLI for noobies: import, display, mogrify”, see  in the Bibliography for further information.