You can have a pretty business woman using an iPad with your app on this stage. Your mockup image in the iPad can be 1024x768px.
Set your website mockup up in a warm and elegant office. Your website image would be places in a Macbook Pro that allows for images of 1280x800px.
Use this black iPad mockup shown in landscape view in a business environment to showcase your product. You can enter a 1024x768px image in this iPad mockup.
Use this iPhone 5c mock up with your best business app. This gives you the perfect office setting for your image.