You can use this website:
https://xem.github.io/3DShomebrew/tools/image-to-bin.html
As long as the images are the exact sizes they need to be (400x240 on the top screen, 320x240 on the bottom), it should work. It's important that you leave the 90 degree rotation on and not preserve transparency, or they won't work.
To install them, name the top screen to splash.bin and the bottom screen to splashbottom.bin. Then put them in sd\luma. If they don't show up, make sure you have splashes enabled in the select-on-boot menu (it's suggested you set them to before).