I'll say first of all that none of my Sims 2 stories use Photoshop, or any other editing program. All the effects are achieved wholly within the game.
However, many cheat codes and hacked objects are used to set up particular shots. Boolean properties are my friend. (Most of the Sims 2 cheats start with boolprop and end with true/false. The so-called "uber"-cheat is
boolprop TestingCheatsEnabled true, which puts the game into debug mode, and lets you do many things that you can't normally - but it's dangerous if you don't know what you're doing. Some of the options are positively hazardous and can result in your game exploding in a Big Firey Ball That's Visible From Space).
Just about every hack in my game comes from the excellent More Awesome Than You!, the Sims site that is so fantastic, we all pay $39.99 US per month to access it. (And if you believe that, you'll believe anything). With J.M. Pescado's amazing game fixes that remove severely annoying behaviour and TwoJeffs' Buyable Career & Aspiration Reward Object sets & Movie Maker Tools, I can do a helluva lot. The other program I use is SimPE by Quaxi, which is occasionally necessary to do things like "fix" relationship stats to allow or disallow certain actions.
Any further questions? Ask away!