Q. When I go to Edit > Define Brush Preset in Photoshop, the option is greyed out and I can’t select this as an option.
A. The most likely explatation is that the image you are trying to define as a brush is over 2500 x 2500 pixels. 2500 pixels square is the maximum size for a brush in photoshop. If you make it 2500 x2500 pixels or less, you should be able to select the Define Preset Option in the Edit Menu.