I posted this earlier this morning on another website -- mirroring it to here
If this is your current header image code:
<img src="http://www.EXAMPLE.com/img.jpg" style="width:227px; height:112px;" alt="Hello World!">
To be mobile friendly (such as on Android or an iPhone), update the style tag to this:
<img src="http://www.EXAMPLE.com/img.jpg" style="width:227px;
max-width:100%; height:auto;" alt="Hello World!">
The magic is that section which has been bolded
