If automatic resizing of uploaded images in WordPress isn’t working, try installing and enabling php5-gd on your Linux server.

To install and enable php5-gd on Ubuntu to enable automatic resizing of uploaded images in WordPress, open a Terminal or connect to the server by SSH and type:

$ sudo apt-get update
$ sudo apt-get install php5-gd
$ /etc/init.d/apache2 restart

WordPress resizes images at the time that they are uploaded, so if you have already uploaded some images, you’ll need to upload the images again or use a plugin like the Dynamic Image Resizer.

A similar process works for other Linux distributions.