Open the photo you want to use. I am using the picture of two cute girls in a pumkin patch I got offline at http://www.betterphoto.com/gallery/.
Add a new layer below the photo by clicking the little icon at the top left of the layers palette, outlined in red below:
Drag the new layer beneath the photo layer. This layer will soon become a mask, so it must be on bottom of the layers stack for this function to work. Select the photo layer and from the Layers menu at the top of your screen, choose Group with Previous.
Your layers palette should look like this:
Don't panic!! Your picture is not gone. Continue on to the next step to fill back in the part of the photo you want to see.
Back in the Layers Palette, select the new blank layer at the bottom and then click on the Brush tool . From the options toolbar at the top of your screen, click the pop-up brush menu, choose Thick Heavy Brushes:
and pick the Rough Round Bristle brush.
Mouse over to your document and paint in the photo.The photo only shows through the area that was painted:
To make the edges more visible, I added a another new layer and dragged it to the bottom of my layers stack like we did in Step 2 above. Select the Eyedropper tool and snatch a nice color from your photo. This will be your new background. Fill the new layer with the color by selecting the paint bucket tool and then pouring the color on the layer. This is my finished photo: