Xara Xtreme for Linux (formerly known as Xaralx) is an open source image editor based on the Windows Xara LX (which is also an image editor, obviously).
Phatch (works on Windows, Linux and Mac OSX) is a simple to use GUI Photo Batch Processor which handles all popular image formats. Phatch can batch resize, rotate, apply perspective, shadows, rounded corners, and more. Also, it allows you to use EXIF and IPTC tags for renaming and data stamping.
A few months ago I was telling you about sK1, an open source vector graphics editor similar to CorelDRAW, Adobe Illustrator, or Freehand.
The release of GIMP 2.7.0 is a first step towards GIMP 2.8, the next stable release. Please note that this is an unstable development snapshot and read the GIMP 2.7 release notes. Actually, in the following repository, the current version of Gimp is 2.7.1 already. To install it in Ubuntu, just copy & paste the following commands into a terminal to add the Launchpad PPA repository:
GIMP, despite it's interface, is the best open-source image editor. But often all you want to do is simply resize a picture or a similar minor task and for that you could use an application such as mtPaint which has a lot of great features such as: