How does nginx perform different operations based on the response of the proxy server?

There is a problem. Some pictures exist on the local server, and some exist on third-party servers.
And the picture names are similar ~ it’s hard to distinguish ~
So I want to configure a proxy. If it can’t be found on the local server, proxy it to the third-party server. Third-party server.
How to configure this nginx?

---

Final solution:

<code>location /{
        root  xxxxx;
        index index.html;
        if (!-e $request_filename) {
            proxy_pass yyyyyy;
        }
    }</code>

Reply content:

There is a problem. Some pictures exist on the local server, and some exist on third-party servers.
And the picture names are similar ~ it’s hard to distinguish ~
So I want to configure a proxy. If it can’t be found on the local server, proxy it to the third-party server. Third-party server.
How to configure this nginx?

---

Final solution:

<code>location /{
        root  xxxxx;
        index index.html;
        if (!-e $request_filename) {
            proxy_pass yyyyyy;
        }
    }</code>

Error redirect

location /{

<code>error_page   404   @test;</code>

}

location @test{

<code>proxy_pass http://upstreamName;</code>

}

Finally solved this~

<code>location /{
        root xxxx;
        index index.html;
        if (!-e $request_filename) {
            proxy_pass yyyyyyyyy;
        }
    }</code>