Pre-installed movies and photos on new MBA
I just made the following test : 

 sudo find /system -name "*.png" | wc -l


returns 4553 on my mac : There are over 4000 images in my /System/ directory. It's probably your case too.  

Using 

sudo find /system -name "*.png" -print0 | xargs -0 du | awk '{total+=$1} END{print total}'


I get 248352K, that 250MB. Not your 450MB but we are getting close. You can assume the files you see are system files (yes your system uses images and video !) 

I would definitely not suggest to delete theses files since they are used by the system.