Thank you for bringing your great question to the community! We have a post in our blog with a helpful infographic guide! Find it here: [Infographic] 2017 Social Media Image Sizes Cheat Sheet. You could always post to social media pages seporately if you would like to use different images for each social media site.
See how to remove a page from a post:
I also have a guide I'd like to share in case you are unfamiliar with how to edit an image within Constant Contact. This article specifically details Facebook image dimensions but provides step by step instructions on how to resize in the Library: Optimal Image Size for Social Share.
I hope this helps answer your question, otherwise, please reply and let us know!
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