The nice people at Sprout Social have set up this super handy Google Doc that has the image dimensions for all the major social media platforms.
The document is set up with tabs for each major platform so it’s really easy to use, and they are keeping it up to date every time information is released about image sizes changing. Facebook are particularly prone to changing their image sizes, so this is a really useful resource that will help ensure your campaign images look their best.
One note however: I learned the hard way recently that the advice Twitter gives on their own profile cover images is not quite right. This tutorial from TwelveSkip shows exactly where to place your message and branding so that it will display properly on your Twitter profile.