Discussions of Images and Symbols
Note: this advice was originally designed for readers writing about three Hawthorne short stories, but it works just as well for medievalists. Actually, since the habit of symbolic thinking was inherently medieval and Hawthorne was resurrecting it in American literature in a self-conscious attempt to become a serious artist, it's more likely that you'll find symbolic uses of images than not in a medieval text. The trick is to support the claim that its symbolic referent is what you think it is, rather than some other one.
Some students are intrigued by Hawthorne's overt (or nearly so) use of symbols like a character named "Faith" in "Young Goodman Brown" or the black veil in "The Minister's Black Veil." If you really want to compare symbolism in two stories rather than just tracing the function of symbols in one, you need to restrict your focus to one or two symbols per story or the paper will become unmanagably large. Remember that not all images (or sounds or feelings etc.) are symbols. A "symbol" must "stand for something else," of course, but all words "stand for things." Symbols stand for special things, things not otherwise present, or things not otherwise detectable. For instance, a cross might be a symbol for a death, a sacrifice, the Christian religion in general, Jesus in particular, or just an intersection in a highway. Context must be brought into evidence in order to say "X is a symbol of Y." One of your best pieces of evidence that "X" really is a symbol of something might be if a character in the tale (including the narrator) says that it is a symbol or speculates that it might be. Otherwise, your burden of proof is much higher.
Look for evidence that the thing you're looking at acts like the thing you think it stands for. For instance, in Arthurian narrative, Galahad is a type of Christ-symbol (virginal, pure in intent, self-sacrificing) who saves a land laid waste by a terrible misdeed (the "Dolerous Stroke"). When his bodily ascent into Heaven leaves his father, the sinful Lancelot, resolved to be a better man, he could be said to have had a redemptive effect even in that sense, as well. The accumulation of evidence based on actions congruent with those of Jesus in the NT creates the case for interpreting Galahad as the author's intentional symbolic reference to Jesus.
Remember, too, that if many different things seem to "stand for" or "suggest" the same kind of thing, you probably are looking at a thematic image rather than a single "symbol." Thematic images repeat (the "theme" part) in various forms the kind of thing the image is about (e.g., images of shadow and light, images of innocence, images of animals). The repetition of the thematic image is designed to remind the reader (sometimes subliminally) that some kind of idea is repeating in varying places (disguise and revelation, purity [and the threat of corruption?], appetites or degradations of the human form, or perhaps only innocent nature vs. human sophistication). Just as in the case of the symbol, you have to provide evidence from context (e.g., perhaps, another story?) to support your relation of the image to the function you say it serves.