Create grayscale masks in Blender like Gimp I want to create an image only with highlights in Blender using the following method:

Image -> increase lightness = Image 2

Image 2 -> covert to black and white = BW-Mask 

apply BW-Mask to image 2 like in Gimp to get Image 3 than contains only highlights and the rest of the image is transparent.

Thanks in advance...
All you have to do is to multiply the image by a variable using the Converter --> Math node (This will automatically convert to BW).

Then you use the Converter --> Set alpha node while using the same image as the mask.