Define Brush in Photoshop Greyed out??

[mashinhead]

I'm trying to define a brush in photoshop. I opened my image, made it greyscale, adjusted the levels, i have a white background with black and grey midtones, but the define brush option is greyed out. i mad a bunch like this yesterday and it was fine.

 

[aldog]

what are the dimensions of the image? the max size for a brush is 2500 px.
Also if it's set as a background layer change it to just a normal layer, this might make a difference
And make sure you don't have the transform tool selected or something like that, make it so the marquee tool is active and surrounding the whole picture.
hope that helps