One of the best functions we can use in Fireworks is Mask. It's a basic command that can make our life easiser when working with images or when we need to hide some part of an object. In this tutorial we will create a Polaroid photo using amazing and inspirational illustrations from Adhemas. We already talked about him, but if you don't know the work of this Brazilian artist yet, please check out his site after the tutorial!
But for now, let's take a look at how this command works.
STEP 1 - Open a new file
Open Fireworks and choose a 600x600 size for canvas.

STEP 2 - Creating the paper
Our first element will be a Rectangle with 400x500px and color #DDDDDD. After creating it apply some shadows and a texture like the image.

STEP 3 - Creating the canvas
Now you will create another rectangle. This will be the limit of our photo. So draw a white rectangle with 345x370px and place like the image. Apply some shadows to create depth in the paper.

STEP 4 - Choosing the image
As I said before, we choose work with Adhemas images (that is bringing a lot of value to the tutorial). Select the image you want and place inside the file. We will now use 2 techniques to make the mask. They both are very easy and will take us to the same result.

STEP 5A - Applying mask
In the first one, first select the image, place above the white rectangle, and Cut it (Ctrl+X or Command+X). Now select the white rectangle, and paste pressing Shift (Ctrl+Shift+V or Command+Shift+V). You will see the image inside the rectangle and at the same position that it was before.

STEP 5B - Applying mask
In this method, we will first place the image inside the file. After that, move the image layer above the white rectangle (just to see the rectangle to select it!). Now let's cut the rectangle (Ctrl+X or Command+X). With the rectangle in the clipboard, select the image, go to Modify > Mask > Paste as Mask. You will see that the rectangle made a mask in the image, the same way that we made it before.

STEP 6 - Playing with the mask
This step is mostly a Tip! If you look at your layers, note that the layer of the image has a mask. If you unlock this mask you can move the image and the mask separately. This way you could place your image with the exact focus you want.

FINAL RESULT
Now you can play with the mask. Use your imagination, explore backgrounds, textures and whatever else you want.

