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diff --git a/tools/image_generator/README.md b/tools/image_generator/README.md
index 5d70354e4..1230ad510 100644
--- a/tools/image_generator/README.md
+++ b/tools/image_generator/README.md
@@ -19,3 +19,13 @@ emulators with different dpi.
 4. `resourceDirectory`: The resource directory that contains all the translated
    strings in xml format, e.g. bootable/recovery/tools/recovery_l10n/res/
 5. `outputFilename`: Path to the generated image.
+
+# Locales
+Supported locales and background texts are located in
+[tools/recovery_l10n/res/](../recovery_l10n/res/values). For each background text, the tool renders
+a localized image for every supported locale.
+
+Each individual localized image contains an encoded locale header string, and the rendered
+background text. The locale header string is generated by `Locale.forLanguageTag`. And sample
+result include `en-US`, `zh-CN`, etc. These individual images are then concatenated together to
+form the final resource image that locates in res/images, e.g. `install_text.png`