I don't think I have ever seen a picture of this, least not afterwards, I've seen some of during the process, but it's not something I have bookmarked.
Yes the bright yellow stuff is normal, don't worry about that, it will go away on it's own as well.
Sleeping is normal, on some cats the anesthic can keep hold of them for a loooong time. Others snap right out of it.
Drinking water is ok, he's been through a big ordeal. So long as say a week from now he's still not drinking excessive amounts.
I'm not sure why his anus would look like that, maybe they ran some test or took his temperature, or perhaps something else. Personally I would wait at least another day or two before letting him back outside. If he doesn't do #2 by tomorrow night, you might want to start to worry, course that's including that he has been eating and drinking well. I know some cats and dogs will go for 3 days every now and then, even on a normal basis with out going #2. Keep your eye on him. If he doesn't go tonight, he'll probably go after breakfast tomorrow morning.