Hi, I am using AddThis to share my post in Facebook. When I clicked the share, I have the below url: www.xxx/Users/Five-ways-to-feel-better-when-it%20feels-like-life-sucks#.WMoLHk8WReE.link
and I used the Facebook Open Graph debugger to scrape the URL.
It returned :
<meta property="og:image" content="www.xxx/~/media/YouthSG/UGC-Five-ways-to-feel-better/Five-things-teaser.ashx?w=300&thn=1">

How do I share my post in FB without resizing the image size? The image size to shown in the share window as what the user uploaded. Is it controlled at our end or Fackbook will resize the image for us?

Hello, the image URL you provided is not K2's default.
You can choose which image (size) is used in K2's settings.