To create a border for picture with musemage mask will be pretty easy. In this tutorial, I will show you how simple it is.
This is the original picture: 
And the following is the effect: 
Now, let’s get started.
Step 1: Copy the original picture as a new layer. And then, open a picture which you want to make it as background.
Step 2: Click the copy layer, and then click Move Up. After that, click Free Transform Tool to make the copy layer smaller. You can use this tool according to your actural situation.
Step 3: Click the background layer, then add a mask. Next, choose Brush Tool, and set the parameters as below.
Step 4: Paint an area on the picture as you like, and click the copy layer and Move down. Thus, the background is manifested. You can modify the border by yourself again.
Step 5: Add some decorate material to attach some color elements.
Open a decorate material as a new layer, and use Free Transform Tool to make it a proper size.