I’d like to customize my robot, something like displaying something on the screen or similar.
I’d also like to know how to do it.
One thing you can do is display an image on the brain screen with an SD card or a C array. Plenty of sources in the forum for how to do that. 
You can use an SD card to display an image or gif on the brain screen.
For gifs: GitHub - jpearman/vexcode-gifdemo
Alternatively, you can add addressable LEDs, but that’s a little bit harder.
try this site: VEX Robotics Image Converter