Milton is an Open Source painting program that's easy to use and allows you to "Just Paint."
There are no pixels; you can paint with (almost) infinite detail. It feels raster-based, but it works with vectors.
The application is under 2 megabytes and simple to use. There are still quite a few options, including backgrounds, eraser, brush options, layers, opacity, and more.
There is no save button to worry about as your work is persistent, and you can undo at any time.
XnView Full is a powerful tool that enables users to view, process, and convert image files easily. XnView Full has all plug-ins and languages included. XnView Standard has just a few add-ons included.