Vectr (www.vectr.com) is a vector-based drawing program. If you're not familiar with vectors, this means that you draw each shape or line individually and you have continual control over the location, size and position of each shape. As opposed to conventional bitmap-based drawing, where each shape is a collection of pixels that can't easily be manipulated in isolation from all the other pixels on the page.
Although you can download Vectr and use it on your PC, there's really no need to do so because there's an online version that works in your browser. When you've created your masterpiece, you can then download it to your computer in a variety of formats.
Vectr is free to use online, though you'll need to sign in with an account such as Facebook.