The threads on it are sort of confusing as most people doing this assume a higher-than-beginner level of understanding with it. I had never done anything like this before but I was able to finally figure it out for myself and here's what I did:
You'll need an a program to see the files on your iPhone. I use(d) DiskAid (I think you can get a free trial, if not it's $10)
Here's how I do it in DiskAid:
Connect phone to computer
Open DiskAid
Click on "Root" on the left side
Then click on "User"
Then below "User", click on "Applications"
Each application folder will be a huge series of numbers and letters. Click on one and you'll see this (This is an example of Wolfenstein 3D game. See the circled area? That's what you'll be working with):
Take the "iTunesArtwork" file (the 93.1 kb one), and right click it to copy it to your desktop.
Now, click on "wolf3d.app"
You want to look for "icon" (or something similar) - in this case, the icon file would be "wolf3d_icon.png" - this is the file you want to replace with the new resized one you are going to make
Now, go to your desktop and right click the "iTunesArtwork" file and click "rename", and add ".png" to the end of the file name (so you actually rename it as "iTunesArtwork.png"). Now you should be able to open/edit it, and this is when you resize it. I used Paint.net to resize them. Resize them to 114x114 pixels.
Once you have resized it, you want to save it as "wolf3d_icon.png", and replace the original "wolf3d_icon.png" with your new one.
What I did is just right click the original "wolf3d_icon.png" file and delete it. Then take my new "wolf3d_icon.png" from the desktop and drag it right into the old one's place. Once you do that, respring your phone and enjoy your new retina display icon.
If you accidentally replace the wrong file or something, just delete the app off your phone and re-download it from the AppStore
