Some themes (Twenty Twelve, Responsive…) use the builtin WordPress custom header image. That’s a nice feature, but what if you include text in this header image? You will need one image per language! It is very easy to obtain thanks to the wpml-config.xml file.
Step 1: create a directory ‘polylang’ in ‘wp-content’ (this is a place where you can store Polylang cutomizations which will not be overwritten when updating Polylang).
Step 2: Copy paste the following code in a file called ‘wpml-config.xml’ that you upload in your ‘wp-content/polylang’ directory:
<wpml-config>
<admin-texts>
<key name="theme_mods_twentytwelve">
<key name="header_image" />
</key>
</admin-texts>
</wpml-config>
You need to replace twentytwelve by the name of your theme (in fact the name of your theme directory)
Step 3: Set an header image in ‘Appearance -> Header’
Step 4: Go to Settings -> Languages -> Strings translations. You can now choose one header image url per language!