As adjectives the difference between superadditive and subadditive

is that superadditive is (mathematics|of a function) such that the image of a sum is at least the sum of the images of the summands while subadditive is (of a function) such that the image of a sum is at most the sum of the images of the summands.

As adjectives the difference between image and subadditive

is that image is figurative (of sense of term or discourse) while subadditive is (of a function) such that the image of a sum is at most the sum of the images of the summands.