I Googled "rpg maker plugin loading screen" and got a bunch of threads you can read through. This one appears to have some specific information:
I have seen a few threads on how to replace/edit the default loader in RMV and else but no luck with RMZ. I have found that this time it's CSS instead of a GIF or APNG, so basically what is the right way to replace that with an apng? or is that not ideal and I should code a custom CSS loader...
forums.rpgmakerweb.com
Which engine are you asking for? Your profile says you use MV, but you posted in the forum for MZ. The code may be different between the two engines.