Any program that works in Layers (Gimp is a free one) will work. Simply put the picture on a bottom layer, create a new empty layer on top of it, and fill it (the empty layer) with your color of choice. That will hide the picture under it. Make the layer 50% or so transparent so you can see the picture under it. Then, just take your eraser tool and turn it into the shape of a circle and start cropping out parts from the colored layer, avoiding any clothed areas. Once you're satisfied, change the opacity (transparency) back up to 100% on the colored layer, then merge the 2 layers and save the image.
It's actually really simple to figure out just looking at the picture. The technique is called "Bubbling" I think.