If you're on a windows, though, the controls may be somewhat different.
With the knowledge of a good photo editor (or even MicroSoft Paint), that's achievable.
1. Make sure you are on the screen you want.
2. Press the Print Screen button. On some computers it's abbreviated to Prnt Scrn (or something like that) and on most, it's also the SysRq button.
3. Open a photo editing software and paste in the photo.
4. Crop the image so it only shows the portrait.
This works with other things, so it should work for Sims 3!