Vegan Gaymer

By the way, I finally got exactly what I needed. Maybe this will help others trying to get the same insight.
Using an image that is 1280 X 853, the viewable parts of the image would look like this:

Thanks for working this out!
This is what bugs me about IPS, trying to find out what should be basic information like this or for that matter the recommended header background image dimensions (that bizarrely in 2017, we admin's still can't upload from the front end unlike a calendar event cover image or profile image!), is often like extracting blood from a stone. 
It should be in the guidance section or FAQ for heaven's sake! Companies for instance like Twitter do it for their recommended header image sizes, because they know users will benefit from getting it right, and their product looks better as a result. 
I'm having a bad day, ignore me.

∞ x 140 should cover it. But that's only the page header, when looking at the All Clubs page a different height/width is used...

Stop.  Just stop with your semantics.  You gave bad advice and no amount of wordplay will save you.  

That is not true, as there can be size recommendations.  Banner ads over the years follow standards in regards to size based on the intended placement.  They range from small boxes (for almost anywhere) to large/wide banners (sides or top/bottom of pages).  So if there can be standard sizes for ads, there can be size recommendations for club banners.
 
Try to keep in mind that some of your members may be using a screen width that is smaller than what you use.  I recommend looking up standard sizes for advertising banners and picking one that you feel will suit your needs.  Then set that as the recommended size for your site.  You wouldn't have to make it a rule that images must be that size, just no bigger than that size (though if you do that, you might want to set a minimum size requirement).  You also might want to make sure the image remains the same size regardless of the size of the visitors screen, with the image being set to be centered, so even if the image is smaller than the max size, it won't look out of place.

That would depend on the resolution of your screen or the fixed width of your forum template.
On the default skin with no side bars the image is 1309x140.
On the default skin with no side bars, set to a fluid width of 95% in Google Chrome on a 1920x1200 screen, with the page maximized the image is 1776x140.

Thank you, @FGN !! I may opt in, myself!

As a cloud user, I think I can answer this, 
Yes it is, but you have to send in a support ticket to get it....
I have and they are supporting it good for me, works good and I can't wait till the final comes out.
Just know that half your apps won't work, as all of the authors are waiting till stable release comes out to update their apps and plugins

Is the beta available for cloud suite?

Wow this is amazing.
Sorry if this has been asked already, but will there be an option to migrate from the Group Collaborations addon?
So excited about this new feature!!!!