-Here's what you can do:
-Put the splash file on your SD card. (Anywhere is ok, as long as you can find it, or in the luma folder.)
-Open your SD card with GM9 and copy the splash file.
-Return to the partition selection, and open "1:"
-Open /rw, and then /luma, then paste the file in. It'll ask you to unlock writing to Sysnand, so do the key combo.
That should get the splash how you want it, although, I think you need to put it on the SD card too (In the luma folder.) so it'll show if you boot with the SD card it. I don't have a splash on mine, but there shouldn't be a reason this doesn't work.