I didn't know that every Mac comes with a simple paint program built-in, until I asked for advice on getting a new one. It's part of the Preview application. Here's how to access it.
How to
Copy an image to the clipboard.
Launch the Preview app.
Choose New from Clipboard from the File menu.
When the image window opens, click on the toolbox icon to enable the editing features. Screen shot.
You should see a row of icons.
What I was looking for
I use a paint program basically to edit small sidebar images for my blog and for screen shots to document my software. These are the features I need.
Resize an image, usually scaling.
Make the background transparent.
Draw an arrow on the image.
Save as either GIF, JPG or PNG.
The Preview app can do all this and more. And the other features don't seem to get in the way.
I'm still figuring out how to fit it into my workflow.