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There isn't an optimal dimension, due to the fact that we don't all access the internet via PCs with 960px monitors anymore and your images will be resized by your browser to fit whatever the screen size of the device is.
If you are just putting in as series of images to illustrate your mostly text post, don't go huge, it is distracting. I recommended 3-400px above.
If you are posting a single or series of photographs, go for 6-800px, which is sizeable but not an overwhelming file size.

CHeers @nikv and @teamhumble- some of the image sthat I have been posting have been pretty big, but very HQ as well, it's a trade- off between load times and happy followers for me. My posts are all about the pictrures.

From an aesthetic point of view, I have it when the edges of the pictures don't line- up in the feed...

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