Checking if a transitive key dependency is in 3rd normal form I'm currently trouble determining if one of my schemas is in third normal form.

I have a table with columns image_id (A), upload_time (B), uploader_id (C), and image_file (D). A is a key (unique identifier for every row in the table), and BC is also a key (every pair of uploader and upload time is unique). Thus, we have A -> BCD, and BC -> A (and transitively BC -> A through A -> BCD). 

Is this table in 3rd normal form? I'm not entirely sure, because I know 3NF allows for a partial key transitive dependency (e.g. XY -> Z, Z -> X), but I'm not sure if it allows for a total key transitive dependency like I have above.
A simple check to see if a relation schema is in Third Normal Form is to see if every left hand side of non-trivial functional dependencies is a superkey (and this test can be used also to check if a relation is in Boyce-Codd Normal Form), or, if this is not the case, if every attribute on the right part is a prime attribute (i.e. part of any candidate key).

So in you case both the dependencies A → BCD and BC → A (and also BC → D) have a candidate key as left part, so the schema is in Third Normal Form (and in this case also in the more strict Boyce-Codd Normal Form).