I came across this great infographic and wanted to share it with you. It highlights all the sizes you need for the various social platforms. View it now.
To make it easy for you, I have broken it down so you can print this page as a very easy tip card.
Facebook
Profile image: 180 pixels wide by 180 pixels tall
Cover (big image for the header): 851 pixels wide by 315 pixels tall
Images you share in the content should be 1200 pixels wide by 900 pixels tall (width is most important)
Twitter
Profile image: 400 pixels wide by 400 pixels tall
Cover (big header image): 1500 pixels wide by 500 pixels tall
Images you share in the content should be 440 pixels wide by 220 pixels tall (width is most important)
Google+
Profile image: 250 pixels wide by 250 pixels tall
Cover (big header image): 1080 pixels wide by 608 pixels tall
Images you share in the content should be 497 pixels wide by 373 pixels tall
Note: on the images you share in content, I would not resize my images to fit these dimensions, but if you are buying images for use, you need to buy big enough versions so buy the 1200 pixel wide version as when you load the image to Twitter and Google+ it will be resized to fit properly but too small of an image will look bad in your content or may not even be able to be loaded.
For Pinterest, Tumblr and other social media sites make sure to check the infographic.