Using a backslash doesn't work? I escape periods in my rewrite's just like so \. with no problems. What about using QSA instead if it doesn't work? If I remember correctly you should be able to do

RewriteRule ^image$ image.php [QSA, L]

and it should work like /image?img=image.png => /image.php?img=image.png. Worst possible scenario you could do

RewriteRule ^image/([A-Z0-9]+)/?$ image.php?img=$1 [NC, L]

and site.com/image/image.png would pump it through your php file. According to this
though you should be able to escape with \... but I tried it and it wasn't working either, so I'm lost.
-m