Is there an image editor like Windows' Paint for OS X?
You could try Paintbrush (free). As quoted from its website :
Paintbrush is a simple paint program for Mac OS X, reminiscent of Microsoft Paint and Apple’s own now-defunct MacPaint. It provides users with a means to make simple images quickly, something which has been noticeably absent from the Mac for years.
Acorn ($49 or free with reduced features)
Acorn is what you want.
Pixelmator is a great alternative to Photoshop and similar to paint.net on windows