Ok, so I renamed the imagedisplay.php file to imagedisplay.jpg, and saved the .htaccess file with the AddType declaration, but I still get a 500 server error. Do you know why this could be?
It's because the hosting company was prepared for people like you who wanted to use this trick. We never said that parsing JPG files for PHP would work... we just said that if anything was going to work, this had the highest likelihood of working.
One last thing you might try: rename your PHP file again, give it an extension that you make up (maybe .PXP or maybe .john1704 or something like that) and then change the .htaccess file again with NogDog's command:
AddType application/x-httpd-php .john1704 .pxp
It might work. If not, I'd consider hosting your web site at one of the $8 per month hosting companies that has PHP.