I'm pretty sure it's perfectly ok to modify the loading screen since the engine is all open source and the devs are super friendly.
I modified the loading screen by messing around with the html and css files of the exported project. You can also change the loading image by replacing the image file. It's pretty simple to do if you know a bit of web-programming.
I've uploaded my old html and css files, in case you want a look / template to go off of.
Let me know if you need more help, good luck! :)