I tried the following which seemed to work ok on my system:

RewriteCond %{QUERY_STRING} ^[a-z0-9]{1,20}\.(?:jpe?g|gif|png|bmp)$ [NC]
RewriteRule ^image$ img.php?img=%0 [L]


One thing to note is that your PHP file shouldn't be named the same as what comes before the query string, ie image?test.jpg should not use image.php because it appears that Apache's content negotiation gets in the way and serves up the PHP page without going through the rewrite process. Simply rename the PHP page to something else as I have in my example above, img.php