Allow php to parse png files via htaccess problem

On my local machine I have apache set up to run png's with this line:

AddType application/x-httpd-php .php .php3 .php4 .phtml .png

in the httpd.conf

I have a file that is named with the png extension but is a gd function to make an image.

All works well and the browser gets the image and a right mouse click to save as shows the filetype and name correct.

On my web server, to which I don't have access to httpd.conf, I edited my (blank) .htaccess file to include:

AddType application/x-httpd-php .png

But got 500 errors. Am I doing anyhting wrong?

Trevor