不应该这么来弄，应该是结合lua/njs来做，通过njs/lua来判断文件是否存在

error_log stderr notice;
daemon off;
events { }

http {
include /usr/local/openresty/nginx/conf/mime.types;

server {
listen 80;

location @image_server {
content_by_lua_file "file_judge.lua";
}

location ~ ^/images/(?<sig>[^/]+)/(?<size>[^/]+)/(?<path>.*\.(?<ext>[a-z_]*))$ {
root cache;
set_md5 $digest "$size/$path";
try_files /$digest.$ext @image_server;
}
}

}

-- make sure the file exists
local file = io.open(source_fname)

if not file then
return_not_found()
end

file:close()

lua里面去判断文件存不存在