Nginx : images not served in HTTPS

I have on my website an admin/ subdirectory, which I'd like to be in HTTPS, so I tried the following configuration, based on this one :

server {
    listen 80;

    server_name blob.tld;
    root /srv/www/blob;
    index index.php index.html index.htm;

    location /blog/admin/* {
        return 301 https://$server_name$request_uri;
    }

    location / {
        try_files $uri $uri/ $uri/index.php /index.html;
    }

    location ~ \.php$ {
        fastcgi_split_path_info ^(.+\.php)(/.+)$;
        fastcgi_pass 127.0.0.1:9000;
        fastcgi_index index.php;
        include fastcgi_params;
    }
}

server {
    listen 443 ssl;
    server_name blob.tld;

    root /srv/www/blob/;
    index index.php index.html index.htm;

    ssl_certificate /srv/www/blob.tld.pem;
    ssl_certificate_key /srv/www/blob.tld.key;

    ssl_session_timeout 5m;

    ssl_protocols SSLv3 TLSv1;
    ssl_ciphers ALL:!ADH:!EXPORT56:RC4+RSA:+HIGH:+MEDIUM:+LOW:+SSLv3:+EXP;
    ssl_prefer_server_ciphers on;

    location /blog/admin {
        fastcgi_split_path_info ^(.+\.php)(/.+)$;
        fastcgi_pass 127.0.0.1:9000;
        fastcgi_index index.php;
        include fastcgi_params;
        try_files $uri $uri/index.php /index.html;
    }

    location / {
        return 301 http://$server_name$request_uri;
    }
}

But then images in admin/style/ are not served.

I looked at the log files, that say :

/var/log/nginx/access.log:
127.0.0.1 - - [25/Apr/2014:15:06:27 +0200] "GET /blog/admin/style/lock.png HTTP/1.1" 403 46 "-" "Mozilla/5.0 (Windows NT 6.2; Win64; x64) AppleWebKit (KHTML, like Gecko) Chrome/32.0"

/var/log/nginx/error.log:
2014/04/25 15:06:27 [error] 23629#0: *404 FastCGI sent in stderr: "Access to the script '/srv/www/blob/blog/admin/style/lock.png' has been denied (see security.limit_extensions)" while reading response header from upstream, client: 127.0.0.1, server: blob.tld, request: "GET /blog/admin/style/lock.png HTTP/1.1", upstream: "fastcgi://127.0.0.1:9000"

Given the error.log file, I think the problem comes from the first location instruction in the HTTPS server (the difference with the HTTP one being ~ \.php$). So I tried to make the exact symetric (with \.php$ instructions in another location instruction) :

server {
    listen 443 ssl;
    [...]

    location /blog/admin/* {
        try_files $uri $uri/ $uri/index.php /index.html;
    }

    location / {
        return 301 http://$server_name$request_uri;
    }

    location ~ \.php$ {
        fastcgi_split_path_info ^(.+\.php)(/.+)$;
        fastcgi_pass 127.0.0.1:9000;
        fastcgi_index index.php;
        include fastcgi_params;
    }
}

But then… no HTTPS at all.

I still have the solution of letting images be served in HTTP, but that is kinda frustrating :

location  /blog/admin/style {
    return 301 http://$server_name$request_uri;
}

I have nginx 1.1.19 and php 5.3.10 with php-fpm.