Using Image Hoses with white edges

First, choose your primary Image Hose.  This is done by clicking the
large image in the brush attributes floating window.  If this window
is not visible, select the Window Menu bar, and select "Show Brush
Settings".

Select a circular Image Hose with white edges, such as this one:



This image hose should create a square on the canvas.  To turn this
image hose into a round, inverted image hose, select the Image Hoses
menu, and "Invert Current Image Hose":



To leave the colors of this brush mostly intact, you can also mix this
brush with a black and white round Image Hose (see the tutorial
#1 How to choose and mask a brush for more
info).