.tif to brush

Hey..Is it possible to make a brush out of a black and white .tif file? i tried everything i knew.. converting it to jpg, changing the layer mode etc, but "define brush preset" remains inactive. I have some really nice splashes of colors in tif file, which i would like to convert to brush. how do i do it?
thanks

You have to select the area you want turned into a brush shape. But it only makes a brush shape based on the luminosity values of your defined area. Colours from the selection are not applied to the brush.