In which important scenes did Dickens use visual descriptions and what are those descriptions symbolic of?
Wow. Dickens uses description everywhere in this novel, so it would not be possible to discuss them all in a forum such as this. Instead, I'm going to pick one which will give you the idea of description being symbolic.
Chapter 5 is the reader's first introduction to the Paris suburb of St. Antoine and the Defarges' wine shop. The people are oppressed and hopeless; the city is oppressed and hopeless. He begins with a description of a broken wine cask and the desperate citizens who are literally lapping the dregs from the mud--symbolic of the time when blood, too, would be spilled. (Dickens is quite helpful in this regard, as he follows up the description with a paragraph beginning "The time was to come when...." This is his cue that what he just wrote has symbolic significance.)
My favorite picture of the town's desperation is his description of the trade signs. He calls them "Illustrations of Want." The butcher's sign depicts "only the leanest scrags of meat." The baker's shows only the "coarsest of meagre loaves." The wine shop's sign has customers hunched over "their scanty measures of thin wine and beer." The only businesses in "flourishing condition" are the tools, weapons, cutler's knives, and smith's hammers--the instruments of a coming Revolution. And, true to form, Dickens follows that paragraph with the words "For the time was to come when...."
You'll find similar descriptions and foreshadowings when he introduces anything or anyone new: the Shoemaker (Dr. Manette), or Monsieur the Marquis, or the rude farm carts which will one day be transformed into the tumbrels carting people to the guillotine.
Dickens just isn't too subtle when it comes to letting the reader in on the bigger picture--the foreshadowing and symbolic nature of the persons, places, and things of this novel. Once you start looking, you'll find it everywhere.