Monkey Raptor

So Twitter Summary Card, in the old days, recommended square sized image.
I think (correct me if I'm wrong).

Nowadays, it'd be cropped.
When I looked again at the Twitter Card Validator, the image container on the summary card has 120px x 90px in dimensions.

Using super sweet Algebra, I simplified the ratio into 4:3.
4 for the width and 3 for the height.

For instance, you already have an image with 300px x 300px size, then you need to stretch the canvas (not the image) of the image. Like so:

Initial canvas size

• Width: 300px
• Height: 300px

Stretched canvas
We need to stretch only the width of the canvas.

• So, new width: (4/3) * 300px = 400px
• Height: 300px (the same).

We need to keep the image dimension intact, so it won't look weird.

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Result

• On Twitter (the summary card), it will be displayed as (or shrunk to) 120px x 90px in size.
  
• On Google+, using the 4:3 ratio, it will be shown quite awesome. So you might want to make your image at least 300px high, so it won't look blurry.
  
• Facebook will also display it like Google+ does. Huge.
  

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How to stretch just the canvas?
I use the magnificent very earlier version of Photoshop here.
On Photoshop, you can click the Image menu ► Canvas size ► switch the dimensions in pixels ► tinker the new canvas size.

Then Save for Web & Devices ► I'd recommend using transparent png format for that.

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On your site/blog
Upload your new image to your file server/cloud ► then update your website/blog image thumbnail link/source. In the meta tag.

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While I was typing this
I searched for Twitter Summary Card documentation, and yes, it will be shrunk to 120px x 90px. The minimum dimension is 120px for height and width.
Thus, use 4:3 ratio if you don't want the thumbnail getting cropped (for Twitter Summary Card).

Link on Twitter Developers

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References for website/blog meta tags

1. Twitter Card Overview
2. Facebook Open Graph: Using Objects

Search engines (Google, Bing, DuckDuckGo, Baidu, Yandex, Goo, and all) don't need any special meta. The official super crawlers will collect the matching Twitter/Facebook tags, plus your interesting contents of course.

Other social medias probably follow what Twitter/Google+/Facebook has for the social sharing display.