Monster Manual: What details?

[the Jester]

And if the players know the creature's image, is that really a problem?

In some campaigns, yes.

All this "you'd know an orc from a goblin before you went on your first adventure" talk is also true- in some campaigns.

Some campaigns, nonhuman monsters are rare and mysterious. Some campaigns, the dm wants to keep the players (as well as their characters!) guessing. This is where awesome things like the players fighting things that they call "dog gobblers" or "stink wiggles" instead of "goblins" and "otyughs" comes from.

 

[Mattachine]

. . . and, in those campaigns, the DM can describe the creature in his own words, based on the good illustration in the MM.