"Digital photos are large files."
Translation:Les photos numériques sont de gros fichiers.
I still don't get why I need the 'de' in some of these sentences. It doesn't belong with the verb in this case I don't think.
In English we can say either "large files" or "some large files". The "some" part is optional in English (and often omitted) and it is mandatory in French. It is "des" immediately before a plural noun, and "de" otherwise, e.g. before a plural adjective. "de" combines with "le" to become "du", and "les" to become "des". Both contractions are mandatory.