How to display front and back of a mock-up image with code?

[ZakThreads]

When you hover your mouse over a mock-up image it will show you that image in, let's say black, then red, but instead I want the mock-up to show the design of the product image's front and back. What code would I need to use and where would I put it?

[ZakThreads]

Need a picture, no problem.

[ZakThreads]

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