I'm guessing this case has hard drive mounting cages that can be removed and re-installed. For example, the bottom right metal cage looks like you can remove it, install 2 or 3 hard drives in it, then re-install. That is how you gain access to the space between this cage and the front panel. I can't tell from the picture whether that space is large enough tor a 120 mm fan, or only an 80 mm or 92 mm. When mounted this way, the fan blows fresh air over your hard drives to cool them; the air then flows into the rest of the case, especially towards your two video cards.
You may have to remove the front cover, also - the one that is blue with slots in it for air intake. This just gives you better access for turning screws into the fan corners to mount it. There may also be a place in there for a foam air filter that should be mounted on the fan intake side, between the metal front panel and the plastic outer piece . This filter should be removed and cleaned from time to time.