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Hi @heyimsnuffles !

For Instagram : the first photo (or the thumbnail of the video) becomes the thumbnail on Steem. The problem is that if you edit your post, the json_metadata of your post, generated by the share2steem app, will be written upon by the Steem front-end you use and will re-put the "avatar" as the Steem thumbnail.
So I recommand that if you edit your posts, remove your avatar (or relocate it) and the divider.

For Twitter the system is the same, but I must confess there are still some problems in the formatting process (Twitter is much more complex in the different types of format and text it can receive so harder to reformat for Steem after that and cover all the cases possible). But I'm working on it.

For the more general reformatting : maybe the avatar and link to the feed will be put to the end. We are waiting for more feedbacks / returns.