In Windows I use an a free image resizer called PIXresizer and it is excellent. Unfortunately it does not run under Linux and whilst GIMP can be used for this purpose it is not particularly convenient.
If you run Nautilus as your file browser i.e. your are using the GNOME window manager, then you can add an extension called Nautilus Image Converter. It’s available in the Ubuntu Software Centre, just search for “Nautilus Image Converter” or you can do an sudo apt-get install nautilus-image-converter in Terminal.
Once installed you will need to restart Nautilus by either logging out and then back into Linux or by entering nautilus -q in Terminal.
When you open Nautilus and right click on an image(s) file you will see two additional menu options:
The same options are also available under the Edit menu in the menu bar.
Selecting Resize Images.. will give you the following window.
One you have finished adjusting the settings, click Resize and your selected image(s) will be resized.