New to posting, If I have an image that's higher quality than an already uploaded image, Is there a way for me to upload the higher quality one without it getting flagged for duplicate post, do i have to do anything extra?

You can message the mods to replace the old image but it won't be easy.

Related question, what do I do if there are several duplicates, all of different resolutions, but the largest is absurdly large (like over 10k pixels on one side). Do they still delete all but the largest? Images that large seem inconvenient.