Using Raspberry to switch 12V
There may not be enough current available on the pins 2&amp;4 "GPIO" (actually power pins). The Velleman relay should draw less than 100mA when on. 

The +5V is drawn directly from the USB port with a resettable fuse in series. Looks like a 2A fuse on yours (which means it might trip at 1.5 or 1.3A at elevated temperature). The current is also limited by what the USB can supply. So if you look at those two limits and take the smaller you may get some idea.  

There is insufficient information on the Velleman to determine if it will work reliably from a 0V/3.3V signal. No specification and no schematic. 

I would guess it might work well enough for you but that's a guess. If you hook it up correctly it shouldn't hurt anything, but you may not be able to tell if it is marginal.