i18n - Translate only specific fields
The best way to do so is by configuring the multilingual parameter of your node type as "Enabled, with field translation". 

The steps : 


enable the module i18n_fields
edit your node type
go to publishing options tab
set Multilingual support => "Enabled, with field translation"


This way, there will be only one node for all the translation of your node (e.g.: node/534) and only the translatable fields will change between language.

You need to then define which fields you want to translate :


in the content type, go to manage fields tab
choose the field and edit
check the box "Field translation" at the bottom