If the game itself have to have some screenshot command in the first place then for 99% those games will not be supported:
- BlazBlue: Continuum Shift II
- Dead or Alive Dimensions
- Mario and Sonic at the London 2012 Olimpic Games (however I will re-check this one)
- Tales of the Abyss
- Tekken 3D Prime Edition

It's a bit sad we can't upload other image-based stuff, however it has one little explanation (a bit overdone tough)
Maybe Nintendo did it this way so we won't accidentally upload photos that would put us in embarassing/difficult/&c. situations or allow others to do so
After all Nintendo 3DS is console for everyone...