Correct me if I'm wrong, but it seems that you previously used WordPress' built-in custom header feature, available under Appearance > Header in your dashboard. This theme feature makes your custom headers available in your theme and in Jetpack's Mobile Theme.
When you switched to a different theme, that option disappeared, and consequently also disappeared from the Mobile Theme.
To bring it back, you have 2 solutions:
- You can add custom header support to your theme, as explained here:
http://codex.wordpress.org/Custom_Headers - You can customize Jetpack's Mobile Theme to add an image in the header. To do so, you can use the Custom CSS module as explained here:
http://jetpack.me/2013/06/27/customize-mobile-theme/#throughthecustomcssmodule
I hope this helps.
