Thanks to my friend, Pam Brossman, I can report that the image that goes in the timeline should be 1200px x 441px for it to fit beautifully.

The timeline picture does crop to fit, but if you are including some words in that image, then I recommend using the above dimensions.

Enjoy!

PS  If you need someone to craft a great advertisement for your timeline wall, let me know and I can put something together for you at a reasonable rate.