Transparent Box
A transparent box can look great on a simple layout. You can write a text over it or just simply use it as decoration. I will try to explain how to make a transparent box on this page.
Step 1: Firstly, find a picture you want to use, I'll be using this one:
Step 2: Now use the
rectangle tool, which looks like this:

to make a rectangle on your picture in the size you want(you can edit the rectangle afterwards if you're not happy with it). You also can move the box around and put it where you want it.
Step 3: Change the
opacity of the rectangle layer to around 50 or 60%. Add some text and decoration and you're done! Here's my finished result: