If WordPress show a message error “Fatal error: Allowed memory size”, for example to add a picture or installing a new plugin in your wordpress account. This is because you are exceeded memory limit reserved to PHP and you need increase it. I propose different solutions.

Solution at WordPress error «Fatal error: Allowed memory size»

Login via FTP client (Filezilla for example) to your hosting account and try to solve the problem
with one of the following solutions:

1. Edit wp-config.php file, the file is in the blog root folder and add at the end define(‘wp_memory_limit’,’XXM’); where XXM is the amount of memory you need:

   /** WordPress absolute path to the WordPress directory. */if ( !defined('ABSPATH') )
   define('ABSPATH', dirname(__FILE__) . '/');
   /** Sets up WordPress vars and included files. */require_once(ABSPATH . 'wp-settings.php');
   
   define('wp_memory_limit','80M');

   If you don’t solve the problem, try with define(‘WP_MAX_MEMORY_LIMIT’, ‘XXM’)

   /** WordPress absolute path to the WordPress directory. */if ( !defined('ABSPATH') )
   define('ABSPATH', dirname(__FILE__) . '/');
   /** Sets up WordPress vars and included files. */require_once(ABSPATH . 'wp-settings.php');
   
   define('WP_MAX_MEMORY_LIMIT', '80M');

2. Edit .htaccess file, this file is in the blog root folder (probably hidden)
   and add the code line php_value memory_limit XXM:

   <IfModule mod_rewrite.c>
   php_value memory_limit 64M
   RewriteEngine On
   RewriteBase /
   RewriteRule ^index\.php$ - [L]
   RewriteCond %{REQUEST_FILENAME} !-f
   RewriteCond %{REQUEST_FILENAME} !-d
   RewriteRule . /index.php [L]
   </IfModule>

3. Try to increase a memory specified in the default-constants.php file,
   this file find it in wp-includes folder. Edit line 9 where in the example
   is define(‘WP_MEMORY_LIMIT’, ’48M’); and especified increasing the memory you need:

   function wp_initial_constants() {
   global $blog_id;
   
   // set memory limits
   if ( !defined('WP_MEMORY_LIMIT') ) {
   if( is_multisite() ) {
   define('WP_MEMORY_LIMIT', '64M');
   } else {
   define('WP_MEMORY_LIMIT', '48M');
   }
   }
   
   if ( ! defined( 'WP_MAX_MEMORY_LIMIT' ) ) {
   define( 'WP_MAX_MEMORY_LIMIT', '256M' );
   }

   If your wordpress install is multisite, then modify line 7 where in the example is define(‘WP_MEMORY_LIMIT’, ’64M’);