Pencil
It is for draw cartoons. But it is so easy to use.
Also easy to install.(Around 1 Mb to download).
Also can make animations from drawings.(There is a timeline). That
gives a basic skill for Blender.
Can save image in diffrent formats.
sudo apt-get install pencil
** If in Ubuntu 12.04 or older:
sudo apt-get install libming1
Then install from above link with Software Centre
To Save the created picture Go to File Menu and select Export Option. Then Select Image Sequence. If you Want to save it as a Catoon, select, Movie or Flash/swf Option.
Then, if it is a Picture, in Forth-comming Export as dialog, type file name as [Filename].png
or .jpg