For cryptoKitties, the images of the cats are based on gene, for example, color, curve type etc. and the image of the cat will be created, how does the image created, it is by programming running background? does it mean the server keep a lof of different types of tails, ears, mouths and use a program to combine them? what kind of program can do this? Thanks
I'm not familiar with the implementation details, but yes. An off-chain service generates the images from the 32-byte "DNA" held in the token contract. The image is subjective in that it is an interpretation of the DNA.
I find myself frequently citing this example of architecture that nicely separates on-chain and off-chain concerns. The on-chain footprint is minimal.
Hope it helps.