If you look in the error log from your site

2015/01/25 13:01:31 [error] 7477#0: *11490 client intended to send too large body: 1424254 bytes, client: 2a02:168:66b9:0:dc3c:5449:d617:86f6, server: www.safematix.com, request: "POST /wp-admin/async-upload.php HTTP/1.1", host: "www.safematix.com", referrer: "https://www.safematix.com/wp-admin/post-new.php"

Open the nginx.conf and edit the http tag

 vi /etc/nginx/nginx.conf

 http {
 client_max_body_size 50M;
 ...
 ...
 }

It is also possible to put the settings per location tag.

location {
 client_max_body_size 50M;
 ...
 ...
 }

Don’t forget the reload from the nginx service.

service nginx reload