try
`AddType application/x-httpd-php .html`
in your .htaccess file
Or even
`<Files example.html>
AddType application/x-httpd-php .html
</Files>`
to make it just process PHP for a specific file.
Alternatively, you can put rewrite rule in .htaccess to internally redirect the call to the .html URL to your .php version.
`RewriteRule ^example.html$ example.php [L]`
If you can't change your .htaccess, or your host doesn't let use AddType or rewrites, then you might be stuck. The only other thing I could think of would be to get the HTML page to redirect to the PHP pages -- either with a meta refresh tag or some document.location JavaScript.
And as an absolute last option, you can have a random image picked and displayed solely by JavaScript. This would requiring you updating the page each time you want to change the list of possible images it picks from, though.
Lots of options