If you are only interested in taking screenshots for your personal use you can use the C key to take a picture just press C in CAS screen or in live mode or Buy mode, and you can find the picture in Documents/EA Games/The Sims 2/Storytelling. Make sure you find the largest size picture, the thumbnail will do you no good. You might want to delete all the Maxis pictures there first, so you can easily find your screenshot.
If you are taking your pics for an upload either at mts, or another site, you will need to use a printscreen program to take your pics. I use Gadwin Printscreen You should download the regular Gadwin Printscreen, the Printscreen pro will do you no good, because it's a trial for one, and another reason is I just personally find it easier to use the regular Gadwin Printscreen. To use Gadwin printscreen you will also need a graphics editing program such as Gimp all you do is open Gadwin printscreen and then in the Destination area, you can browse to your gimp.exe or the gimp application. When you get in your game press F10 in live, buy, build, or options screen to hide the UI. You can't hide the UI in CAS screen, but need to crop it in gimp. Then press your printscreen key on your keyboard. Once it opens in gimp, you can then use the rectangle select tool to select the areas of your screen you want to crop. Then you can go to Image/Crop to selection. Now you will probably need to resize the picture. Go to Image/Scale Image, and bring the scale to something around 800 x 600. Most sites require that your pictures be no bigger than 800 x 600 pixels in size. Then go to file/save as at the top and browse to where you want to save your screenshot. I usually just stick it in my pictures folder.
A lot of time it depends on what program you're using. If you ARE using just the in-game camera option, they're in your Neighbourhood -> Storytelling folder. 3rd party programs: I use Fraps, and it saves on Drive C: in its own folder.
If you do end up using a third-party program, I highly recommend doublechecking which key it defaults to for capture: Fraps started out with F10, which of course meant that every time I wanted to take a screenshot, my UCP disappeared/reappeared (and vice versa). I had to re-assign it to F9, which holds no in-game control.