If you regularly need to resize and/or rotate images, you can add a right-click menu item to the Nautilus file manger by installing the nautilus-image-manipulator and nautilus-image-converter packages:
sudo apt-get install nautilus-image-manipulator nautilus-image-converter
then restart nautilus to activate the plugins:
nautilus -q
Now just right-click any image, and you will have extra options to resize and rotate the image 