For this tutorial you will need two images, the first of the building and the second of some dripping paint.
On the right hand side click the button Composition.
Resize and move your image to where you want it (click on the Move Resize and Scale button on the left).
Now go carefully around your paint removing all areas that need removing. It is a good idea to be zoomed in when doing this.
Also reducing the Opacity is good to see exactly where needs removing.
The last step I have done is to add an effect to the whole image.
Click the Effects Gallery button on the top right hand side, you can narrow down your search by the options on the left.
This is the end result,to try out for yourself download a free trial now.