Make a new document any size, for this one I made it at 1680px x 1050px.

Select a brush any basic one. I choose the splatter 59 pixel. Basic brush that comes with photoshop.

Create a new layer, if you don't know how there's a arrow pointing to the icon you click to make one.

Now on layer 1 [layer you just made], Make any type of shape you want.

It can be as random as the one you see on the screen shot.

Now create two more new layers.

Go on to your layer 2. As highlighted in the screen shot.

Now on your table that has your layers and such in. There's a arrow that looks somewhat like a play button?

Its right under the exit "x" click on it and as seen in the screen shot click on the "create clipping mask" [ alt + ctrl + G ]

See how there's a arrow now going pointing down to layer 1.

Which means what ever you put in to layer two will only show in the shape you made on layer 1.

Open up the image you would like to use in the shape you made earlier.

For this I will choose a photograph I took a while ago.

Use to the lasso tool and go around the whole image and once you have it press [ Ctrl + C ]

To copy the image.

Now go back to your shape you made, and go to layer 2 with the arrow pointing down.

And press [ Ctrl + V ] and the image should come up. You can use the move tool to position it where you want it to be.