The convert command from the ImageMagick suite of tools provides ways to make all sorts of changes to image files. Among these is an option to change the resolution of images. The syntax is simple, and the command runs extremely quickly. It can also convert a image from one format to another (e.g., jpg to png) as well as blur, crop, despeckle, dither, flip and join images and more.
Although the commands and scripts in this post mostly focus on jpg files, the convert command also works with a large variety of other image files, including png, bmp, svg, tiff, gif and such.
To resize an image using the convert, you would use a command like this:
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Source:: Network World – Linux