Hi,
Here is the behavior you can expect with images according to PrestaShop htaccess "bootstrap" :
- Il the file exists il will be served and PrestaShop won't even be fired. You can try a request on init.php file : you will get a 500 error that shows the index.php (bootstrap file) is not fired.
- If the file does not exist the request will be rewriten in index.php and PrestaShop will be fired (and modules as well)
In your case, the /img/p/pl.jpg (or en/jpg) file exists and you wont't have any redirect. If you delete this file your redirect should work but i wonder why you don't just replace the img by the one you want (?).
Hope it will help.
Yann