The key thing to remember about imagery is that it is not description. Imagery is using language figuratively – simile, metaphor, personification, hyperbole. If you need to talk about description more generally, you could perhaps use semantic field as a concept to make it more technical. Describing the semantic field of a pattern of imagery is also an effective way to use literary and linguistic methods together in an integrated way, and to discuss a text’s construction more cohesively.
For a Lang-Lit example, in writing about Othello, you might comment on Iago’s use of imagery from a semantic field of pollution and corruption, which he uses to cement the idea that Desdemona has been tainted in Othello’s mind.
To discuss imagery in a Language analysis, it might be relevant to explore the representations created by a pattern of imagery, such as violent, feral connotations offered by animal metaphors in a speech about gangs.
The theme of a text is what it’s ‘about’, but not the plot. For example, some themes of Othello are jealousy, gender, truth and deception. Themes tend to be abstract nouns, as they’re often ‘big ideas’.
For both Lang-Lit and Language, you’ll need to apply this idea to texts other than literary texts (as well as literary texts in Lang-Lit of course). Themes do emerge in conversations, interviews and media and non-fiction texts as well as literature.