As the title identifies, I’ll be using the image editor GIMP to demonstrate the Threshold and Color to Alpha tools. If you don’t already have a copy, head over to GIMP.org and download one. it’s free which is why I use it 
.
This is what we’re shooting for:
Threshold
The Threshold tool turns your image into a true Black and White. That is Black and White only, no shades of grey. You have to carefully choose the image you want to use this effect upon. If the image has too many colors or limited contrast, it may not work too well. However, if in doubt, open up the image and give the Threshold tool a whirl and see how it turns out, a little experimentation may find a brand new trick!
So, let’s take my original image:
You’ll notice that this image has a lot of conrtast and limited colors. It’s perfect for turning the Threshold tool loose on. Now, you’ll find the Threshold tool under Colors > Threshold…
I’ve selected the Threshold tool and am adjusting the slider to get just the effect I want. I think right about here gives good detail. Let’s go with that.
Now we have a black and white image, perfect for using the Color to Alpha tool (Alpha is a fancy name for transparent).
Color to Alpha tool
You can find Color to Alpha under Color > Color to Alpha… Let’s choose that now. Here’s a shot of the open Color to Alpha tool. I selected white in the “From … to alpha” box. As soon as I did that, all the white in the image disappeared like this:
Now we have an image that is only black, black trees. The rest of the image is transparent. Lets add a layer and move it underneath these black trees. If you look at the Layers, Channels, Paths and so on window (it’s usually on the right side of your screen looking something like this:
Let’s add another layer. I’ve circled in red the new layer button. This will add a transparent layer above the tree image. This new layer need to be placed below the trees so just click on the new layer and pull it below the original one. Voila, now you have a clean slate to lay in the background!
I chose a Hubble space telescope image for this background. I opened the space image and scaled it down to fit my tree image. Copied the space image, click on the trees again, clicked on the transparent layer and pasted the space image into it. That’s it. All that is left is a quick rotate of the whole image. I decided that I wanted it rotated to the left so I choose Image > Transform > Rotate 90 counter clockwise. Be sure and click on the finished image and view a larger size version. Much of the detail is lost with the small sizes.
Now, let’s see what you can do!
GIMP is an acronym for GNU Image Manipulation Program. It is a freely distributed program for such tasks as photo retouching, image composition and image authoring.
It has many capabilities. It can be used as a simple paint program, an expert quality photo retouching program, an online batch processing system, a mass production image renderer, an image format converter, etc.
GIMP is expandable and extensible. It is designed to be augmented with plug-ins and extensions to do just about anything. The advanced scripting interface allows everything from the simplest task to the most complex image manipulation procedures to be easily scripted.
GIMP is written and developed under X11 on UNIX platforms. But basically the same code also runs on MS Windows and Mac OS X.
