If you want to get curated, I’d definitely add an image. Curation is partly automated. There’s an algorithm that automatically discards submitted stories due to various inadequacies: not in English, too many typos, violation of curation guidelines, etc.
I don’t know for sure, but I suspect that the algorithm wouldn’t look too kindly at stories without any images whatsoever.
As to how long to spend on an article, well, it depends on your abilities, topics, amount of research required, etc. I’m a very slow writer and I illustrate all my stories. So it takes me days to finish a piece. Then again, most of my stories get curated.
If your goal is curation, I’d aim for higher quality and spending more time. At least in the beginning.
There used to be a thing called curation jail (not sure if it still exists), where writers could get blacklisted for producing writing that’s not up to Medium’s standards. Once you were in curation jail, the Medium curation algorithm simply rejected anything coming from you. I’d definitely try to avoid that!
Later on, when you’re able to get regularly curated, you can try to speed up the process and see if you’re still getting curated.
Anyway. Just my 2c :-)