Here's a great reference sheet for sizing all of your images on the major social media sites. Need to design a new Facebook cover photo? Want a custom Twitter background?
Pass along this reference to your graphic designer and bookmark it for future reference.
Social Media Dimensions Quick Reference
Facebook Cover photo: 851 x 315px
Facebook Profile photo: Displayed at 160 x 160px, but the recommended upload size is 180 x 180.
Facebook Event Page cover photo: 784 x 295px.
Twitter Header image: 1500 x 500px,max file size is 5MB.
Twitter Profile image: 400 x 400px, but you can try uploading photo at 800 x 800px so it doesn't lose quality.
Google+ Cover photo: 2120 x 1192px.
Google+ Profile photo: 270 x 270
LinkedIn Cover photo: 646 x 220px
LinkedIn Profile photo: 100 x 60px. The square logo is 50 x 50px.
Pinterest Profile Photo: 160 x 165
Instagram Profile Photo: 110 x 110
YouTube Cover Art: 2560 x 1440px for TV, 2560 x 423px for Desktop.