Some tests I did on blog post banner sizes as discussed on 2016.1 Deep Dive: Content
[Please ignore all the apostrophes - I'm mortified ]
All blog posts look like this one - the banner was selected to be vertically central (Centered). The title of the blog post shows the size of the banner used in pixels. [Other girl bands are available at your record store and online.] Edit: or as I learnt today! YMMV.
Then I looked at how the blog posts appear in the activity stream. Where there is colored padding there is a thin black border on the padding so that you can see that there is no clipping going on:
As you can see, 'so far' there is no clipping, so you can use an aspect ratio which is heading towards portrait as long as the 'important bit' of the banner is appropriately positioned based on how you align the image on the blog posts (Top Center, Centered, Bottom Centered).
What the blog posts look like in Jive Daily:
Jive Daily stream pics (sorry not in logical order):
Blog posts on a tablet
1280 x 400 in (tablet) portrait mode on iPad - note cropped at the side (and we're not talking about Frankie Sandford):
1280 x 1500 in (tablet) portrait mode on iPad:
1280 x 400 in (tablet) landscape mode on iPad:
1280 x 1500 in (tablet) landscape mode on iPad: