Does anyone know of a simple Image Editor, with functionality comparable to Paint.NET, for Ubuntu? I've always found GIMP to be overkill and too complicated for what I want to do.
Are you looking for something like TuxPaint?
mypaint is also great and simple. It is in the software center.
You might also want to check out Krita or for plain photo editing digikam with its plugins.
Both are packaged in Ubuntu. Also for kids related drawing tuxpaint is great.
There's Pinta as David points out, which clones the user interface and took the same open sourced code to implement image adjustements and effects. There are other lightweight alternatives that might fit the bill if you don't require features such as layers or filters: Gnome Paint and KolourPaint for KDE.
The Gnome Paint site hosts a DEB file, so you can download that and double click to install. To install KolourPaint:
sudo apt-get install kolourpaint4
Or search for it on the Software Center. Note that if you don't currently have any other KDE app, then installing KolourPaint will also install other KDE specific libraries that will take a considerable amout of disk space. That being said, I found KolourPaint more feature complete than Gnome Paint.
I'm not sure what you mean by "simple," but a good web alternative that can be run on any recent broswer on any operating system is the Aviary suite. They have a lot of different programs for different tasks, so instead of like gimp with just about evry function aviary sorts them out and they also work together. So if you need a different set of tools from another programin its suite u just have to press a button. This is the website, it is powerful but the interface is relatively simple