This is a cool effect and it is really easy to do. I suspect that it could also be done in Elements and perhaps Gimp etc.,
My file is 700 × 500 @72 dpi, you will have to adjust some of the sizes to use this effect on a larger file. You can nick my image below to practise on if you like … copyright is mine yada, yada.
With the/your image open choose the shape tool from the tools palette and make sure that the Rectangle Tool is selected. Make your foreground colour black. Draw out a rectangle on your image similar to the one below.
In the layers palette, and making sure that you are working on the ‘vector mask’ thumbnail by clicking on it, take the Fill of the shape layer down to zero – this will leave you with a 1 pixel border. See image below.
Still on the same layer, go to the FX icon at the bottom of the layers palette and click on Blending Option to bring up the palette. Go to Stroke and change the colour to white, change the size to 10 px and choose ‘Inside’ from the position drop down menu. At this point you can also go and give your rectangle a drop shadow if you want. See images below for settings.
Go to the tools palette and select the polygonal Lasso tool, and keeping just inside the white area, select that area.
Then choose the ‘create new fill or adjustment layer’ from the bottom of the layers palette and choose Hue/Saturation. Take the Saturation all the way down to zero, you could also choose to adjust the lightness slider if you wanted.
Obviously we don’t want this to happen, so making sure you are still on the Hue/Sat adjustment layer press Ctrl + i on your keyboard or go to Image>Adjustments>Invert.
You should still be on the Hue/Sat layer, so shift + click on the shape layer to highlight both of them, then right click and choose ‘Link Layers’ or you can go to Layer>Link Layers in the top toolbar. Bring up the Transform Tool by using the keyboard shortcut Ctrl + T and move the selected pieces around till you have something that suits you or the image. Your highly unlikely to choose the transformation I did though 🙂 LOL
Because the Shape and Hue Sat layers are linked you can move them around independantly of the background layer to highlight the portion of the image that you want to be in colour. I’ve put another finished image here that might be a litte more appealing 🙂
Or why not a heart shape ….
Or some other random shape that you like.
Have fun 🙂