[i18n] Ensure section-local image paths fall back to default lang image

[chalin]

Originally posted by @chalin in #4741 (comment)

Hugo is now setup to reuse the default language image -- the exact same file, no duplication (as was the case for static resources a few Hugo versions ago).

Images should not be duplicated. I'd suggest using the default image except where there's too much English text in the image, of course.

[chalin]

Actually, it just works (as I expected originally, but was confused by the ja PR that included an image override). Will keep this open for a little while longer; maybe until I at least add a comment to the i18n docs.