If duplicating the image really is unnecessary then you could still perhaps have the best of both worlds... only store the image once, but have it referenced by different filenames, for the different languages - using mod_rewrite (Apache) and an internal rewrite.
Based on unor's example:
These would be internally rewritten to send a request to your server-side script:
Your script knows to serve the same image for both requested files, perhaps with a simple lookup associative array:
'en/house.png' => 'real/path/to/my-house-pic.png',
'es/casa.png' => 'real/path/to/my-house-pic.png',
This method allows you to serve different images for certain files if you wished.