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This is weird, but it seems that Tableau Server just steals 16 pixels from each side when converting to image. I tired it on our test server and got the same result.
I have no idea what causes it, but a simple hack would be to just add 16 pixels to each side and be done with it. Set you dashboard to 693x436, and when you export it as image you'll get 677x420. I tested this and it worked fine.
It seems like an odd place to be stealing pixels and I couldn't figure out if it was resizing or just cropping it in. I've added the extra and its working fine.
It looks like Tableau just scales it, without cropping anything. This very well might be a bug. Are you reporting it to Tableau Support?
I should do that.