The optimal size of your online image depends on how you are using it.
Consider that on one platform, Facebook, a different size is recommended for a profile image versus a photo that anchors an event announcement. Your cover photo should be a wide landscape image, but other photos should be more square.
Facebook Instagram Twitter Google+ LinkedIn Pinterest
See the full infographic below to learn what dimensions each social media platform and specific use requires.
Read more here: http://www.prdaily.com/Main/Articles/49e97be8-dd5d-4b7d-87c6-a8f72b1b1179.aspx