You're doing it upside down.
Put this code in your htaccess
RewriteEngine On
RewriteBase /
RewriteRule ^old([0-9]+)$ image.php?$1 [L]
RewriteRule ^admin/(.*)$ ninja-admin/$1 [L]
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^ index.php [L]
The rule for your images is the following
RewriteRule ^old([0-9]+)$ image.php?$1 [L]
It will forward every url like /old123 (where 123 is one or more digits) to image.php?123
EDIT: if you want to forbid direct access to image.php?xxx then you can do it this way
RewriteEngine On
RewriteBase /
RewriteCond %{THE_REQUEST} \s/image\.php\?([0-9]+) [NC]
RewriteRule ^ old%1 [R=301,L]
RewriteRule ^old([0-9]+)$ image.php?$1 [L]
RewriteRule ^admin/(.*)$ ninja-admin/$1 [L]
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^ index.php [L]