Sure, it's easy.  It's best if you use a square image in gray scale.  Size is up to you, but the larger, the more detail you get in your brush.  Save the image somewhere you can find it later.  It can be a bmp or a jpg or a png.

Then create a terrain, and go to your terrain brush library.  You might want to add a new catagory, then click ADD.  You will be prompted to go to a folder.  Go to the folder where you saved your brush and add it.

It's really really easy.  I've been making brushes out of a lot of Knotplot images.

Could be worse, could be raining.