There are several tools available for Image drawing or editing , graphics design: GIMP, xpaint, gpaint, kolourpaint, mtpaint..
GIMP or General Image Manipulation Program is one of the most popular and good graphics editor.
GIMP can do image editing tasks such as resizing, editing, cropping , converting between different image formats.
To instal on Ubuntu, open terminal and type command:
$ sudo apt-get install gimp
Tip: If you are migrating from Windows to Linux then KolourPaint from http://www.kolourpaint.org/ may be a good choice.