Hi Manteuffel
This is how I would do it, may help.
First copy/paste elsewhere the DAT file and work on that one.
TEST RUN
Note TGA position in DAT file.
Extract TGA and delete the TGA in DAT file. Re-pack the TGA in DAT file again.
Now I've proven that works with the "old TGA".
NEW TGA TROUBLE
If I was getting errors with new TGA I now know the "new TGA" is the problem.
One way of having correct TGA is with copy/paste
Note "old TGA" byte size.
Copy the "new TGA" and paste into the "old TGA" you extracted.
The "new TGA" should now have the "old TGA" settings.
re-Check byte size.
Re-pack.
That has usually worked for me.
Try that for now
p.s. I usually use "Gimp v2" to do this.