Ok, then you should be able to use the APEC boot bin as Ldr.bin to dump your nuvicam's region 16. We'll take it step by step for you. First you'll dump region 16, then modify the dumped 16.bin with your preferred compatible images and finally rewrite it to rgn 16:
Spoiler: Files and 'How To'
1. Extract this rar file directly to a CLEAN microSD card: [Only registered and activated users can see links. ]. Ensure the unit is fully OFF, not just in standby. Insert the card and turn it on. When it reboots you will have '16.bin' in the Garmin folder of the card, make a copy of the bin file to work with and then move that entire 'Garmin' folder somewhere safe as a backup because we'll need the card empty again to use for the write back.

2. Open the copied 16.bin with GIR_Editor and change the splash images as you wish, ensuring the new images are the exact same properties as the existing.

3. Clean the card again, and then extract this rar file to it: [Only registered and activated users can see links. ]. Open the Garmin folder and place your 'new' modified 16.bin file in there. Remember it must be in the Garmin folder itself, alongside the 'Updater' folder.

4. Repeat the process of inserting the card, when it reboots this time you should see your new images on the splash screen.

A further reminder that this is entirely at your risk. I'm relying on the boot bin from APEC version being compatible with your TWN version. Ask first if unsure of anything.