I know, but I explained poorly. That was useful info for other readers, 
I’ve been researching a bit, and there was a Python script in the Codeaurora Android sources (which is the basis for FP Open): https://source.codeaurora.org/quic/la/device/qcom/common/tree/display/logo?h=LA.BR.1.2.7_rb1.1 (python logo_gen.py newsplashimage.png should make a splash.img somewhere)
We can maybe look for the actual script used for FP2 in the https://code.fairphone.com sources
Edit: I couldn’t find anything, logo_gen.py script doesn’t exist in device/qcom/common, and splash.img is included as a binary on the device tree already, 