You can resize photos which are too big. If you have a photo which has a large file size, resizing it will also reduce the file size.
Using the GIMP Image Editor
The GIMP Image Editor supports several methods of resizing photos. We will concentrate on the simplest method.
Open the photo you want to resize in the GIMP Image Editor
Press Image -> Scale Image...
Adjust the Width or Height as appropriate. The image will be kept to scale
If you want to adjust the width and height independently, press the link button which connects the Width and Height boxes, so that it looks like a broken link
Under Quality, change the Interpolation to Cubic (Best). This will give you the best quality resized image
Press Scale to resize the photo
Press File -> Save As... and choose a new filename for the photo
Press Save to save the resized photo
See the GIMP Documentation for more information.
Using the Nautilus Image Converter
The Nautilus Image Converter allows you to simply right-click on an image in Nautilus and resize or rotate it.
To install it, open Synaptic or the Ubuntu Software Center and search for "Nautilus Image Converter". You will need to log out and in again before it works.