Use any high contrast image cleaned for tubing.  If you don't know how to clean an image well, use my Cleaning tutorial to learn to do that first or use an image that is already tubed.

Step 1
Select an image or use this one for the tutorial.
Open the tube (if it's a tube, it should already be 16 million colors), check by clicking on Color/Increase color depth;  if 16 million colors in darkened, click it otherwise it is already 16 million colors.
Using the 'image/Resize' menu, check the top sizing (Pixel size), see which number of the height & width is the larger and type in 255 in it's place.  If you already know neither the height or width is larger than 255 pixels, you can skip this step.

Step 2
Use Selection menu and click on 'Select all'.

Step 3
Click once on the image to set selection.  The image is now ready for creating the brush.