What's inside an image; how it's set up, what media files are present etc can be a bit of a mystery. I wanted to strip ALL the roms and media from a couple of big Pie images I got from here. I'm too old to learn a new OS like Linux and old enough to have already been through the Days of DOS and that's enough for me so plan A; buld a Linux machine was not an option.
I went through a few so-called Linux media reader/writers that supposedly work on Windows but was disappointed until I found a good one. Now, I am not associated with this company and as I have said here before, they could be based in Botswana for all I know. The software is Linux File Systems for Windows by Paragon Software. I got a 10 day free trial and that was all I needed to do what I wanted.
You can actually see the folders and files in a Windows Explorer style GUI or it is Explorer, I can't remember now. Anyway, you can copy out the relevant Amiga stuff you want which would be the roms and the media files I guess, then place them in the other image, room permitting. I'm no expert but I can guess you would have to edit a file o two so the software knows about them there but then again maybe not. The other way would be to throw them on a stick and insert it into the Pie under a specific folder apparently and the Pie will copy all the stuff into the right places for you. Not 100% sure on that but reasonably confident that's what you do. I just found this:
There are three main methods of transferring roms: via USB stick, via SFTP, and via Windows (Samba) shares
SFTP or SSH File Transfer Protocol also called Secure File Transfer Protocol is a network protocol that allows you to securely transfer files over a network. Naturally both your PC and Pi will need to be connected to the same network via Ethernet or Wifi in order to successfully transfer your files.
Wired (needs ethernet cable)
Wireless (needs Pi >= 3 or wifi dongle for Pi <= 2 )
There are many SFTP programs out there: Windows: WinSCP, Mac: Cyberduck are two of 'em.
Samba-Shares
Samba is a software suite that allows you to access file systems over the network. Naturally both your PC and Pi will need to be connected to the same network via Ethernet or Wifi in order to successfully transfer your files.
if on windows type RETROPIE into the computer folder. You can also replace RETROPIE with your Raspberry Pi's IP address
if on MAC OS X open finder, select “Go” menu and “Connect to Server”. Type smb://retropie and hit “Connect”.
Manually copy files from USB-stick
From RetroPie version 3.0 a file manager is available, it allows you to manually transfer files between USB-stick and Raspberry Pi SD card. File manager can be run from 'RetroPie' Emulationstation menu. Your USB-stick should be mounted in /media/usb.
The directories for the ROM files are located in ~/RetroPie/roms/SYSTEMNAME, where SYSTEMNAME is the short name of the corresponding system.
Good luck!