Rounded corners are a great design trick to have in your pocket and are super easy, whether you want to make your own journal cards or add rounded edges to a photo.
The shape tool (U) in Photoshop has several shape options. Right click on the tool to get the menu and choose "rounded rectangle tool."
Up at the top you will get options for your tool, you can set the radius to anything, the bigger the number the more rounded the corners will be.
Then simply make a new layer and draw your shape.
You can clip a photo to your new shape to get rounded corners on your photo.