Gthumb is a lightweight image viewer and organizer that also has a few limited options of
editing photos and it comes with a very simple user-friendly interface . It offers a certain range of image editing operations suited for digital photographs, such as the change of image hue, saturation, lightness, contrast or the adjustment of colors. It can also crop, scale and rotate images and it features a red eye removal functionality. Manipulated images can be saved in the formats JPEG, PNG, TIFF and TGA.
This application is in the Ubuntu repositories and can be installed by opening a terminal and using this command :
sudo apt-get install gthumb
Shotwell is a very popular image organizer and viewer that comes installed by default in Ubuntu and other Linux Distro's.
With this software users can import photos and videos from a digital camera directly. Shotwell automatically groups photos and videos by date, and supports tagging. Its image editing features allow users to straighten, crop, eliminate red eye, and adjust levels and color balance. It also features an auto "enhance" option that will attempt to guess appropriate levels for the image.
Shotwell allows users to publish their images and videos to Facebook, Flickr and Youtube.
If the Distro you are using doesn't come with Shotwell by default you can install via terminal by typing :
sudo apt-get install shotwell