If you click on the above link, you can see a much larger version of the infographic, but we also thought it was worth sharing here at SEW.
All of the sizes are in pixels (width x height), and everything is up-to-date as of 2 December 2015. 
Please let me know if there’s anything you think we need to add, this is very much a work in progress. Under the infographic you’ll find the text from the infographic which we’ll update regularly when changes are made to the various social networks.
In no particular order, here are the various image sizes for Twitter, Facebook, Instagram, Google+, Pinterest, LinkedIn and YouTube.
Click here for a larger version.

Twitter

• Header image: 1500 x 500 (maximum file size 5MB)
• Profile image: 400 x 400 (displays at 200 x 200, maximum file size 2MB)
• In-stream photo: 440 x 220 (but can expand to 1024 x 512)
• Twitter accepts GIF, JPEG, and PNG files
• Photos can be up to 5MB. Animated GIFs can be up to 3MB

Facebook

• Profile Picture: 180 x 180 (displays 160 x 160 on desktop, 140 x 140 on smartphones)
• Cover Photo: 851 x 315 (displays 640 x 360 pixels on smartphones)
• Shared image and link: 1200 x 630

Instagram

• Profile Picture: 77 x 77 (appears as a circular image)
• Photo Thumbnails: 238 x 238
• Photo Size: 641 x 641

Google+

• Profile photo: 250 x 250 (maximum file size 100MB)
• Cover photo: 1080 x 608 (this is ‘recommended”, although the smallest photo you can choose is 480 x 270, the largest photo is 2120 x 1192.)

Pinterest

• Profile picture: 60 x 60 
• Board display: 216 x 146
• Pins appear in boards with a maximum width of 236, with the height scaled.
• Expanded pins have a minimum width of 600, with the height scaled.

LinkedIn

• Profile picture: 120 x 120
• Banner Image: 646 x 220
• Link upload thumbnail: 180 x 110 
• Image upload: maximum width is 350 with the height scaled to fit.

YouTube

• Profile image: 100 x 100 (this is automatically pulled from Google+)
• Cover image: 850 x 176