How can I scale all images in a folder to the same width?

This is very easy to do with imagemagick. You should be able to install it in the Software Center. I would suggest it for batch processing of images.

The batch resizing is incredibly simple (I tested it with Ubuntu 11.10). Use the following command to resize every .jpg file to 200 pixel width, keeping the aspect ratio:

$ convert '*.jpg[200x]' resized%03d.png

you can maintain the filename by using -set option. Ex:

convert "images/*.jpg[250x]" -set filename:base "%[basename]" "images/new_folder/%[filename:base].jpg"

If you have more files you should use with find

find /folder -iname '*.JPG' -exec convert \{} -verbose -set filename:base "%[basename]" -resize 1920x1080\> "/folder/images/%[filename:base].JPG" \;

This is only scratching the surface of the power of imagemagick. Resizing can be tuned endlessly. For more advanced resizing you have to use the -resize option.

You can limit the resizing to shrinking:

$ convert '*.jpg[300x>]' thumb-300-%03d.png

or enlarging:

$ convert '*.jpg[300x<]' thumb-300-%03d.png

Have look at the geometry documentation to see more options.

For GUI, Phatch "one click is worth thousand photos" is the best for such quick job. It is already in Ubuntu repository.

sudo apt-get install phatch

1. Add Scale & Save item
2. Set scale/resize options
3. Set save/output options
4. Run
5. Set input folder
6. Click batch to execute

Try this:

sudo apt-get install gimp-plugin-registry

Then, open up Gimp and open the Batch plugin found in Filters > Batch > Batch Process.

Select your images in the Input tab, and define the Resize operation in the surprisingly-named Resize tab.

More information can be found here.