It's easy when you know how, and have done it a few times! There's a few ways to do it.
Firstly, in the top left hand corner, there's a magic wand to the right on the second row (That's the magic wand tool), you can use this, and click on a bit on the photo, and it'll select all of that colour/section. Sometimes it's not that good though. So...
Secondly, you could use the lasoo tool (The one to the left of the magic wand), and then click where you want to select, and it'll do it for you. Then as with the magic wand, once the area of the window is selected, you select the brush, and paint away, and it'll only paint in the space which is selected.
Or if neither of those work for you, or can't do it, just get a brush, zoom in, and draw over the window freehand. It's a lot easier with a tablet though.