Open a new file with the size 100x100, then put your image on there and crop the picture however you like.
Desaturate the image via Image >> Adjustments >> Desaturate
Create a new layer on top of your desaturaetd image and fill it with a dark red color (#5F0000 or #7F0000).
Then set the layer to Multiply
Looks okay already and if you want, you can now just add borders/brushes/text to it and you're finished. But I think it's a bit dark, so I'm working some more on it.
Here's a bit more finetuning. There are two different methods for this I usually use.
I'm adjusting the brightness and contrast to bring out the details of the image underneath a bit better. Go to Image >> Adjustments >>Brightness/Contrast and my settings were brightness: +14 and contrast: +40, settings for brightness and contrast can vary a bit depending on the image you have.
For the other method you can work with adjusting the curves. To do so, go to Image >> Adjustments >> Curves. I've put the settings on Channel: RGB, input: 77, output: 80. As with the method above, the exact settings can depend on the image, you might have to use more or need less input/output for getting the image shown better.
It looks slightly different than with the brightness/contrast adjustment. In the end, which look you'll prefer is up to you.
And now all that's left for you is to add brushes/borders/text to put the finishing touches on the icon.
The tutorial also works with dark green (#217F00) and dark blue (00487F), for both I've used method 4a. The image used for the tutorial is from Torchwood.de