I've done that too, and found out that on some servers it doesn't work. Local php.ini files are very easy to block for server admins, and it's a standard setting on most hosting server systems to not allow them.
If you want to see exactly what your site can use, create a file called
info.php and add this to it:
<?php phpinfo ();
Upload that to your server, and go to
www.yoursite.com/info.php and you'll be able to see all of the settings that are available for your site. I'll guarantee that that page says that you have 36MB of memory available.
php.ini file hasn't worked, you can also try to do it in your
.htaccess file. Just add this somewhere outside of the WordPress directives:
php_value memory_limit 256M
Again, there's no guarantee that will work, as it also depends on how your hosting company has their servers set up.
If all else fails you will need to contact your hosting company and ask them for their help. They'll either be able ot help you, or you'll know that you should start looking for a better place to host your site.