Creating a high-quality mockup is a great way to show off your work. It's better than a simple screenshot because it helps your audience envision the work on a real device.
You can use mockups like this as cover photos for your portfolio projects, which increases the production value of your site and makes your work look more appealing to peers and potential clients.
I use Adobe Photoshop in this video, but you can likely use any comparable photo manipulation program to accomplish the same results.