Trying to make single images appear same size on iPhone 4/5/6. Coding?

I will start this off with saying I am not familiar with coding language. I purchased the code for an app I needed designed, and my developer disappeared after the release of the iPhone 6/6+.

There are certain images within the app that need to be a certain size on all devices. For example, an image of a $1 bill must be the same size on all phones to match the size of a real $1 bill. Currently, it is correct on the iPhone 4/4s and 5/5s.

However, when I use the iPhone 6, all of the images are blown up and are much too large.

Can anyone lead me in the right direction of having the code recognize when an iPhone 6 is being used? I do not need this to work on the iPhone 6+.

Thank you!