The process below describes how to create a non-rectangular decal image in Revit as below:

Open a Revit project and zoom into a wall that you are adding the decal to. Then switch the Visual style to Realistic as shown below.

Go to Insert Tab -> Decal Type

On the bottom left side of the dialogue box click New and add a name. Then click Ok.

The next step is to browse to the picture you want to add as a decal.

In our case we’ll use the sample windows image files and the Penguins file in particular.

If you click ok this will add the decal as a rectangular image once you use the Insert Place Decal tool and place the decal on the wall.  See below.

Now the idea is to have a non rectangular image and the way to do this is to add a Cutout to the decal. To do this go back to the Insert Tab -> Decal Types and click the Add  Image as a Cut out.

You’ll now browse to a file that you should have precreate looking similar to the one in the image below. I’ll discuss the way to create this image shortly.

Once you clicked ok the Cut out will be applied and your decal will become non rectangular. See below.

Now to discuss how to create the Cutout image. There are many ways to do this in CorelDraw or Photoshop but I’m only suggesting one using MS paint.

So open MS Paint that comes with Windows. Click File->Open and browse to the Penguins file.

Save the file as PenguinCutout or similar name. Now click the Pencil tool and trace over the outlines of your image.

Now set the brush size larger and colour the outside of the outlines black.

Now Rightclick and whiten the inside.

Do this until you have a monochrome image. You can Zoom in to colour in more accurately. Once done Save and you can use this image as a Cutout in Revit.