How to easily resize images via command-line?

First install ImageMagick via:

sudo apt-get install imagemagick

Open a terminal and run this command:

convert  -resize 20% source.png dest.jpg

It will reduce the size to 20%.
Note that the reduction is not by 20%.
The resulting image will be much smaller, 20% of the former size,
not 20% smaller than before, not much smaller.

You can also specify the size:

convert -resize 1024X768  source.png dest.jpg

You can also use: mogrify command-line tool from the same package.

You want simple?

Run sudo apt-get install nautilus-image-converter, or click nautilus-image-converter .

It adds two context menu items in nautlius so you can right click and choose "Resize Image". (The other is "Rotate Image").

You can do a whole directory of images in one go if you like and you don't even have to open up an application to do so.

You need to restart your nautilus to see new context menus, run nautilus -q and then click the Home folder icon to reload nautilus with the new plug-in.

sudo apt-get install imagemagick

The command mogrify overwrites the original files with the resized images:

mogrify -resize 50% *.png      # keep image aspect ratio
mogrify -resize 320x240 *.png  # keep image aspect ratio
mogrify -resize 320x240! *.png # don't keep image aspect ratio
mogrify -resize x240 *.png     # don't keep image aspect ratio
mogrify -resize 320x *.png     # don't keep image aspect ratio

^{Note: You can add -auto-orient to automatically orient converted images.}

If you're just doing a couple of images, most image editors in Ubuntu (Gimp, F-Spot, etc) will let you do a basic resize.

If you want to edit tens, hundreds or thousands of images, I prefer Phatch. Phatch is a GUI-based batch photo editor that will let you perform a whole load of transformations on images. sudo apt-get install phatch

ImageMagick is good but it's a bit tedious if you don't know the setting names for things. You can very quickly learn Phatch by clicking around.

ImageMagick is the package you want. It contains a number of useful command line tools for this very purpose.

Here's a simple tutorial explaining how to batch resize images:-

mogrify -resize 320x240 *.jpg

After this command is completed, all of the images will be replaced with resized version of themselves. Notice that in an effort to preserve the image aspect ratio, mogrify may not be produce images that are exactly 320x240. To force this to happen, modify the original command to by placing an exclamation point at the end of the desired resolution:

mogrify -resize 320x240! *.jpg

No need to install any new software just do this

convert -resize 50% myfigure.png myfigure.jpg

or

convert myfigure.png -resize 200x100 myfigure.jpg

At the moment nautilus-image-converter does not work in Ubuntu 13.10. Therefore I use imagemagick on the command line, which is very good workaround (at least for me).

sudo apt-get install imagemagick

Keep in mind the difference between these imagemagick tools:

• Mogrify does processing on the same image, it reads file modify file and writes the output to the same file.
• Convert is meant to work on separate images, reads file and modify and write to different file/format. You can also use convert command to use output file same as input file.

I often use mogrify to simply resize multiple images and overwrite the original files. I. e. this command would scale down the dimension of all JPG files to 40% of the original dimension:

mogrify -verbose -resize '40%' *.JPG

GIMP is probably the easiest way, since it has a fairly simple UI for such common tasks. All you have to do is open up your image and go to Image → Image Size and then change accordingly. There are ways to do batch resizing using the GIMP as well, but I don't know them by heart.

Install gthumb. Simple and easy for basic image handling and editing functions - viewer, resizing, cropping, rotate, flip, grayscale, etc with options to save in JPEG, PNG, TIFF, TGA formats.

To install gthumb:

• Open your terminal
• Type sudo apt-get install gthumb
• Accept the changes

You can also use the ubiquitous ffmpeg (or avconv) tool to resize images:

ffmpeg -i image.jpg -s 4096x2048 image-resized.jpg

And if you want really fast JPEG image resizing - try epeg (as mentioned here and there) - which needs to be built from source.