I am looking for a way to add text to my App Store screenshots for iOS. An example of what I'm looking for would be the Google app screenshots on iOS. There are many ways to add a device border, such as mockuphone.com, but is there a simple and easy way to add text to those images, preferably not involving Photoshop?
You can always use the annotation tools in Preview.app. These include the ability to add text boxes which can be positioned above the screenshots or other output image from a service such as the one you mentioned. Alternatively, you can use an app such as Pages to create the images by exporting the pages as an image file.