The idiom having been explained, here's a picture of a broad paintbrush for, as TRomano mentioned, painting crude lines:

![1](https://lh3.googleusercontent.com/EmmX9Mh7mRwvIK-2RjEPdsEdZoBxehVM2EX8GIyotybTpvVkGrHq10PDIJtmHsHAwvl1-dM=s85)

To see why it is *broad*, here's the picture of a *fine* (thin in thickness) one for painting small details:

![2](https://encrypted-tbn3.gstatic.com/images?q=tbn:ANd9GcTNVgJmpmdUs8JGhtG4nKiSWayCZz7-q724tVUN2qLwFQlPYYIX)