I recently created a Facebook Cover Image for my new website Internet Crash Courses, and I noticed later that the text on my first draft got cut off on mobile. The image worked for desktop but not for mobile. See what I mean below:
This was the original cover photo:
And this is the new one:
I ended up replacing the image and changing the verbiage a little bit as you can tell, but the most important thing was discovering the left and right margins. With this template I created, you can position your image and text accordingly so that the important elements appear on both mobile and desktop.
Here's the template:
While you're here, also be sure to check out the rest of the resources in my Travel Blogger Success Kit.