Postcard Puzzle

I bought these postcards in Japan a number of years ago. I just stumbled upon them and wondered about them. They are pretty weird. I like them. The guy with the bling-y collar is saying "gochisou sama," which is what you say after a meal. He does look like he's just become stuffed from a good meal. The other guy is Shin-chan, and it says someting about him laughing - "Shin-chan, please don't laugh," I think it says? I am frustrated because I'm not sure about the verb ending and obviously need to practice Japanese more. Actually, whatever it is, I think the milk is saying it. It would make sense for the milk to ask him not to laugh right now, right? Or does it only make sense because I promised myself I'd go to bed at 10:30 and it's already 11:09?
Anyway, the name on the backs of the postcards is "Shinichi Hoshi." I looked up Shinichi Hoshi online and found that he was a Japanese science fiction writer famous for writing short short stories. I didn't find anything about an artist, which kind of surprised me because I feel like I remember seeing a lot of paintings like these while I was there. It made me wonder, did this scifi writer also make wacky paintings? Or is this a separate person? Anyone know? Anyone? Hello??