During the build process for secondary locales, a locale specific directory will be checked for other files. This allows alternative translated images or movies files (for example) to be automatically used when building an output for a secondary locale.
To use alternative (e.g. translated) image files for secondary locales, place the secondary locale other files in a project sub-directory mirroring the directory containing the original file but including the locale identifier in the first part of the path. For example if you have an image with a file name of images\screenshots\mainscreen.png you should put the translated image for the German locale in "images.de-DE\screenshots\mainscreen.png" or "images.de\screenshots\mainscreen.png" (which is an alternate path using only the first part of the locales identifier which is checked after the full locale identifier).
You do not have to replicate the entire set of other files in the locale specific path - only those files that exist in the locale specific path will be used during the build and any that are not found will use the main file version.
If you want to add the translated files to your project (for source control purposes perhaps), be sure to flag the translated files with the "Exclude from online" and "Exclude from booklet" flags in order to avoid doubling up of files in generated compiled help files.