If I understand your concern correctly, you're looking for render/saving settings to satisfy your two primary desires: to save a great pic and for contest submission. I assume you mean to save in the best resolution you can get and at a compressed size to upload. If I got that right, I would only concern myself with resolution--a great pic to save. I ALWAYS render/save at best resolution, usally tiff format (I refer to it as an archive master). From a pic editior, you can adjust the master and save it separately to meet the other (contest) requirements.