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.
Krita is a good alternative for paint.net, for it resembles paint.net most in term of look & feel (and functions).
I worked with paint.net for years (on Win7), now changed to Ubuntu 18.04 and do work with Krita as I have been used to with paint.net
Do NOT use Pinta. While the original recommendation for this tool might have been true at the time of writing, this does no longer apply. Development for Pinta stopped in 2015, see: [Pinta] (https://pinta-project.com/pintaproject/pinta/releases/1-6)
Pinta is unstable at least for Ubuntu 18.04.
Are you looking for something like TuxPaint?
mypaint is also great and simple. It is in the software center.
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.