Here's a fast and easy effect you can apply to a snapshot if you're looking for a quick way to create a stylish avatar -

There's a few variations, but the entire effect shouldn't take more than a minute or two to finish, and it's very effective.

I watched Pirates of the Carribean again the other evening, so I decided I'll be starting this tutorial with Keira Knightly...

Duplicate the layer, and desaturate the duplicate with a Hue/Saturation adjustment layer. Pull the Saturation slider all the way to the left:

and then hold ( on a Mac) while clicking the duplicate background layer immediately beneath it in the Layer stack. Click "More>" at the top of the Layers palette and choose the option Merge Layers. Now there's a duplicate B&W version of Keira.

Now she needs to be selected - I used the Selection Brush with a variety of brush sizes to paint around her, but you can use whichever selection tool you feel the most comfortable with.

Once the selection is made, open Handy Actions and run the actions in the order indicated (first Invert the selection, then create a layer mask, then create a mask from the selection - you could alternately create the layer mask, then run the mask from selection option, then invert the mask - so many different ways to do the same thing!)

Your Layers palette should look similar to this (I already created a new layer between the original background layer and the duplicated B&W layer, and filled it with blue), Keira is now B&W on a colorful BG!

Now there's a few different things you can do.

For a quick, gritty-looking image, add a Threshold Adjustment to the top of the stack, and adjust the slider to get a balance between white and black pixels.

To make this adjustment layer only affect what's directly beneath it, have it selected in the Layers palette and press + (+):

For a bit more control over the degree of B&W, try a Posterize adjustment layer, and use either 2, 3 or 4 as the setting (of course, experiment with other numbers, too - !)

I used a setting of 2 on the left, below (which gives a result very similar to the Threshold Adjustment above), and a setting of 4 on the right.

Now you can crop your image to the size dictated by the forum where you want to use this avatar, and Edit>Save for Web in the format desired!