You're correct, as per Apple's documentation
images marked with @2x.png are for their high resolution devices (note that those icons should be actual .png files and not just manually renamed to .png).
App Icons required for iPad and iPhone are totally different than custom images you plan to use in your application and you should not mistake both. You can use any kind of images in your app and they do not have to conform to any particular specification of Apple. So the answer to your question is yes, you can use "retina" images in your app.
If you plan to show different images based on devices, you could use media queries as explained in the following thread: Best practices to display images and resize according to device and orientation
the Telerik team
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