outside glow

[allelopath]

I have this:
https://drive.google.com/file/d/1aQ0hkLV...sp=sharing

How do I get it to look like one of these:
https://www.americansouthwest.net/utah/c...nts_l.html

[rich2005]

You need to make a greyscale image and export it to a gbr brush.

White is transparent, black is opaque, greys are semi-transparent.  Greyscale will take the brush foreground colour.

You can use the image of the hand, or paint your own. I used the image
Colours -> Threshold to get it mainly black hand on white. Colours -> Invert to get a white hand. Image -> Scale to get down to brush size.

Make a greyscale image (brush size), white canvas and fill with black noise. Noise -> CIE lch  
Copy / paste the hand over the noise, put in lighten-only mode to give white hand in noise. 
Layer -> New from Visible, paint around the edges in white (remember white is transparent in the brush) to fade out.

Export to your brushes folder ~/.config/GIMP/2.10/Brushes

There will be other ways, but all that as a video demo: https://youtu.be/s7uFOGoEnek  4 minutes

[allelopath]

Amazing. Above and beyond anything I expected, thank you!