Pixeden - Pixeden offers premium resources for web and digital service companies. Among other resources, they have great free files for iOS device mockups for iPhone 6S and 6S Plus, iPhone 5S, iPhone 4S, iPad Air and iPad Pro.
App Icon Template - Designing interfaces, screenshots and icons does not need to be complicated. However, if you want the absolute best resources to create world-class designs, the tools on this site will help you produce pixel perfect images and efficiently export them in all of the file formats you need.
iTunes Connect Properties - Apple specifies the properties to use when creating screenshots and app preview videos, and describes how you should complete the rest of your app's record inside iTunes Connect.
A Picture Paints a Thousand Words: Choosing Screenshots for Your iOS App - This article summarizes key information about making app screenshots.
Ultimate Collection of Free Photoshop Styles - Usually Helvetica or San Francisco font is ideal for creating a clean app interface. However, sometimes a game or particular app will call for text that fits with a unique art-directed style: arcade, sci-fi, whimsical, gritty, or something else. When needed, these Photoshop style presets will save you hours in designing the perfect look for your icons and text.
These 50+ Awesome Photoshop Plugins Will Save You A Lot of Time and Money - If you are a full-time designer, these paid resources might be useful for specific projects. These niche plug-ins can accomplish tasks ranging from turning photos into paintings, to exporting multiple Photoshop layers at a time.