Here's a thought: I don't know if it's entirely stable yet, but just use the first frame of the
Pencil animation program that
Rene (Rasheed) mentioned. Simple tools, exports to PNG, although transparent areas of the PNG may show up as black in some viewers.
With the potential that later you can use it for storyboarding
Assuming you're using Windows (since you mentioned Paint):
Artweaver is a nice freeware paint program. Some of it, particularly the brushes, are possibly overkill for what you asked, but you can ignore most of the brushe settings.
There's a number of paint programs that have older or cut-down free versions.
The older free version (down the bottom of the page) of SmoothDraw
http://www.smoothdraw.com/product/index.htm
Very simple but no fill/paintbucket tool.
The free version of ArtRage
http://www.ambientdesign.com/artrage.html
Nicely designed interface but no shapes or fill/paintbucket. Tries to reproduce natural media, does a relatively nice job.
Free version shows the full interface, but pops up a "not available" box for the paid features.
The free version of Project Dogwaffle
http://www.thebest3d.com/dogwaffle/free/
Nice painting program, slightly quirky interface.
Possibly overkill for what you asked.
Also check out one of it's siblings, the non-free
PD Particles, with extra features for drawing plant-like stuff.
The free version of TwistedBrush
http://www.pixarra.com/download.html
I haven't tried this one.
Regards, Myles.