You can resize multiple image files (jpg/png/gif….) stored in a folder by the imagemagick package. Here is step-by-step guideline:
1. Install imagemagick from Ubuntu Software Center
Or, in the terminal:sudo apt-get install imagemagick
2. Put all your image files in a single directory.
3. Open a terminal and go to this directory:
4. Now, enter following command to resize all of the images to a specific percentage. For examples, for the following command, all of the images will be reduced to 50% of their dimension maintaining the ratio.
mogrify -resize 50% -format jpg *
Where -format jpg specifies: the resultant format will be JPG.
You may also specify width and height by the following command:
mogrify -resize 800x600 -format jpg *
You can easily guess, the resultant images will be of width 800 px and height of 600 px, keeping the original ratio.