Sometimes it can be hard to get a photo like that if the ones fighting are your friends. It's show as two friends instead of a slap fight.
I've got it by catching them slapping and pointing the camera at the ground to where there feet are just barely in the top of the frame and had it register as the slap fight instead of two friends.
You just get a picture of there shoes but it still counts. 
But with most pictures like that trying moving the camera around to get it to change what's it's registering.
Yes, another trick, if you really want to capture the actual fight, is taking several pictures of the same action. A lot of times, once it captures a particular photo, it will default to another possible type.
For example: if your sim's spouse and best friend are fighting, you might have to run through "My Spouse," "My Main Squeeze," "My Friend," and "Two of My Friends" (and possibly one or two more) before the "Slap Fight" option comes up.