My I suggest giving it its own directory then adding this htaccess code so it can be accesses as a PNG filename to prevent any compatibility issues. Also, some AV's flag up images when its not a recognised image extension.
This htaccess will redirect all requests to a .png file to a .php file without renaming anything.
Code:
Options +FollowSymlinks
RewriteEngine on
RewriteRule ^(.*)\.png$ $1.php [NC]
This htaccess will redirect all requests to a .png file to a .php file without renaming anything.