CocoPad

CocoPad is a handy application that allows you to draw or edit canvas, take notes, etc. by simply using four predefined pencils; this easy-to-use tool might come in handy at work to help you write down project ideas or at home for personal interest.

Its interface is simple and customizable as you can add or remove components from the toolbar, change the icon's display, etc. You will be able to set the background image from the toolbar: an image can be blank (white), ruled, squared (similar to a math workbook) and transparent.
The program will offer you access to four pencils to use while drawing and allow you to set the color and width you need for each pencil.

A disadvantage of using this application is the fact that it brings you a small amount of tools to help you edit your paintings; you can only draw images by using your mouse as a free hand or trace lines using the Shift button.

You can always re-size the painting at any resolution you like, though, be careful about the adjustments you make as this type of action can't be undone.

All in all, CocoPad is a useful program that helps you draw amazing paintings and edit images by adding notes.