Ggtkuk

Can anybody explain to me why Voldemort didn't know what Grindelwald looked like?

It's not like there were ages between them – both Grindelwald and Dumbledore were still alive in the era of Voldemort, their duel was famous, the information about it was virtually everywhere: in Hogwarts schoolbooks, in other books, even on Dumbledore's chocolate frog card.

In support of this notion I found some dates: Voldemort is believed to be born in 1926, while Grindelwald's was defeated in 1945. Young Tom Riddle must have just graduated from school. Great dark wizard in the making, I don't believe he didn't keep watch on that confrontation.

And he didn't know how Grindelwald looked like?
When Voldemort visited Gregorovitch and found out that the Elder Wand had been stolen, he forced the wandmaker's memory for an image of the thief and wondered who it might have been. Not until he visited the Godric's Hollow and saw Rita Skeeter's book did he know the mysterious thief was Grindelwald.
How come?

It's like living in the former Soviet Union during World War II and not knowing how Hitler looked. Utterly impossible! Even at the time without the Internet and TV every child knew his face. Photographs, newspapers, books, etc...

And yet Voldemort didn't recognise the greatest dark wizard of his youth. And neither did Gregorovitch! An older man who lived in country where Grindelwald was most powerful, at the time when Grindelwald was most powerful, and he didn't know this wizard? It's like an old man in a Russian village during World War II who got the village occupied and his possessions taken and didn't know afterwards who it was, can't even imagine!
Too far-fetched to be believable.

Or am I missing something?

Grindelwald stole the wand before he started his reign of terror. From the books:

Gregorovitch burst into the room at the end of the passage and his lantern illuminated what looked like a workshop; wood shavings and gold gleamed in the swinging pool of light, and there on the window ledge sat perched, like a giant bird, a young man with golden hair.
_{Harry Potter and the Deathly Hallows (Emphasis mine)}

Now, we know that Grindlewald and Dumbledore were approximately the same age (having been youths at about the same time), and Albus Dumbledore was born in 1881. This means that when they had their famous duel at the end of the War (in 1945), they were both around 50 - 60 years old.

Taking your Russian Village example - it's like the 'old man' in question getting burgled by an 18-year-old, long-haired, un-moustached Joseph Stalin, and capturing a photograph of him. About 30 years later, he sees the new leader of the Communist Party of the Soviet Union - would he recognise him? Then, another 40 years later, his now 50-year-old son finds that photograph in a box in the attic - would he recognise who that was.

The fact that Grindelwald possessed the elder wand was not public knowledge. I can bet that nobody outside of Dumbledore knew this, and Tom had no way of getting to Dumbledore.

Sure, Tom might have known who Grindelwald was, and what he looked like. But he had no way of connecting Grindelwald to the elder wand, which is what he needed Gregorovitch for.

So in summary, he probably did know what Grindelwald looked like, but needed the wandmaker to make the connection between the elder wand, and its thief.

It's also very likely that he recognised Grindelwald in the memory. How else was he to know from a memory who the thief was?