Metaphor - Right after Jones is gone at the end of Chapter 2, the pigs drink the milk and try to brush it off as unimportant. The milk is a metaphor for all the privileges the pigs will soon take for themselves.
Simile - "All that year the animals worked like slaves" (Chapter 6, first line).
Imagery - Squealer, the master of propaganda, in Chapter 5 is explaining tactics.
"He repeated a number of times, 'Tactics, comrades, tactics!' skipping round and whisking his tail with a merry laugh."
Imagery uses the senses to create an impression; here we have sight, sound, and touch.
Paralellism - The commandments are always in a parallel construction, as are most of Squealer's propaganda/threats that Jones will come back.
There are many comparisons in Animal Farm, and Old Major's speech in chapter 1 has several examples. The reader becomes engrossed with the animals' predicament and soon realizes that "the life of an animal is misery and slavery." This is a comparison between the animals' existence and slavery or misery and is therefore a metaphor.
Imagery as a literary technique in itself creates visual pictures using the senses to inspire the reader. A good example from Major's speech is "the very instant that our usefulness has come to an end we are slaughtered with hideous cruelty." This comment leaves nothing to the imagination and although the reader may be used to eating meat and so slaughtering animals is actually commonplace, he cannot help but be moved by this comment (and perhaps feel guilty) which humanizes the animals and makes the practice seem quite barbaric.
Parallelism in literature uses structure to stress important information, and is purposefully quite repetitive with a familiar beat. In Old Major's speech, there is much parallelism as Major must express himself clearly because the animals are not particularly intelligent and his words need to inspire them not scare them. The structure of his speech therefore contributes to the animals' confidence. Here are some examples:
"I have had a long life, I have had much time for thought" (chapter 1)
"We are born, we are given just so much food... we are slaughtered with hideous cruelty."
"No animal in England knows the meaning of happiness or leisure... No animal in England is free"
"He (Man) does not give milk, he does not lay eggs, he is too weak to pull the plough, he cannot run fast enough to catch rabbits." Here, even though the words may differ, the rhythm remains the same, almost like a chant. There is a definite warning contained in these words, as despite all this, Man "is lord of all the animals."
There are many striking instances of a certain type of imagery in Orwell's Animal Farm. One of these occurs late in the novel when Boxer overexerts himself and injures his lung.
When Boxer explains that he hopes to retire and the other animals all leave him to relay the news to the rest of the farm, "[o]nly Clover remained, and Benjamin, who lay down at Boxer's side, and without speaking, kept the flies off him with his long tail."
This example of imagery is in line with many others instances from the text as Animal Farm can be called a book of iconic images (or politically/symbolically charged images) more so than it is a book of detailed physical description. The imagery then is constituted of "images" with moral and political weight, like the one quoted above.
Imagery of this sort populates much of the book, from start to finish, as the animals are presented clearly within their social and political roles on the farm. Another striking example comes at the very end of the book when the pigs are seen to have essentially turned into humans.
"The pigs become indistinguishable from the men who own the neighboring farms, and the animals are no better off than they were under human control" (eNotes).