webserver_sample_2.ino

#include <ESP8266WiFi.h>
#include <ESP8266WebServer.h>
 
const char* ssid = "..............";
const char* password = "................";

String form = "<form action='led'><input type='radio' name='state' value='1' checked>On<input type='radio' name='state' value='0'>Off<input type='submit' value='Submit'></form>";

String imagepage = "<img src='/led.png'>";

// This is a png file (led.png)
const char image[] = {
    0x89, 0x50, 0x4e, 0x47, 0x0d, 0x0a, 0x1a, 0x0a, 0x00, 0x00, 0x00, 0x0d, 0x49, 0x48, 0x44, 0x52,
    0x00, 0x00, 0x00, 0x10, 0x00, 0x00, 0x00, 0x10, 0x08, 0x02, 0x00, 0x00, 0x00, 0x90, 0x91, 0x68,
    0x36, 0x00, 0x00, 0x00, 0x01, 0x73, 0x52, 0x47, 0x42, 0x00, 0xae, 0xce, 0x1c, 0xe9, 0x00, 0x00,
    0x00, 0x04, 0x67, 0x41, 0x4d, 0x41, 0x00, 0x00, 0xb1, 0x8f, 0x0b, 0xfc, 0x61, 0x05, 0x00, 0x00,
    0x00, 0x20, 0x63, 0x48, 0x52, 0x4d, 0x00, 0x00, 0x7a, 0x26, 0x00, 0x00, 0x80, 0x84, 0x00, 0x00,
    0xfa, 0x00, 0x00, 0x00, 0x80, 0xe8, 0x00, 0x00, 0x75, 0x30, 0x00, 0x00, 0xea, 0x60, 0x00, 0x00,
    0x3a, 0x98, 0x00, 0x00, 0x17, 0x70, 0x9c, 0xba, 0x51, 0x3c, 0x00, 0x00, 0x00, 0x18, 0x74, 0x45,
    0x58, 0x74, 0x53, 0x6f, 0x66, 0x74, 0x77, 0x61, 0x72, 0x65, 0x00, 0x50, 0x61, 0x69, 0x6e, 0x74,
    0x2e, 0x4e, 0x45, 0x54, 0x20, 0x76, 0x33, 0x2e, 0x33, 0x36, 0xa9, 0xe7, 0xe2, 0x25, 0x00, 0x00,
    0x00, 0x57, 0x49, 0x44, 0x41, 0x54, 0x38, 0x4f, 0x95, 0x52, 0x5b, 0x0a, 0x00, 0x30, 0x08, 0x6a,
    0xf7, 0x3f, 0xf4, 0x1e, 0x14, 0x4d, 0x6a, 0x30, 0x8d, 0x7d, 0x0d, 0x45, 0x2d, 0x87, 0xd9, 0x34,
    0x71, 0x36, 0x41, 0x7a, 0x81, 0x76, 0x95, 0xc2, 0xec, 0x3f, 0xc7, 0x8e, 0x83, 0x72, 0x90, 0x43,
    0x11, 0x10, 0xc4, 0x12, 0x50, 0xb6, 0xc7, 0xab, 0x96, 0xd0, 0xdb, 0x5b, 0x41, 0x5c, 0x6a, 0x0b,
    0xfd, 0x57, 0x28, 0x5b, 0xc2, 0xfd, 0xb2, 0xa1, 0x33, 0x28, 0x45, 0xd0, 0xee, 0x20, 0x5c, 0x9a,
    0xaf, 0x93, 0xd6, 0xbc, 0xdb, 0x25, 0x56, 0x61, 0x01, 0x17, 0x12, 0xae, 0x53, 0x3e, 0x66, 0x32,
    0xba, 0x00, 0x00, 0x00, 0x00, 0x49, 0x45, 0x4e, 0x44, 0xae, 0x42, 0x60, 0x82
  };
  
// HTTP server will listen at port 80
ESP8266WebServer server(80);

const int led = 13;
 
void handle_adc() {
  float val = analogRead(0);
  server.send(200, "text/plain", String(val));
}
 
void handle_led() {
  // get the value of request argument "state" and convert it to an int
  int state = server.arg("state").toInt();
  
  digitalWrite(led, state);
  server.send(200, "text/plain", String("LED is now ") + ((state)?"on":"off"));
}

void handle_image() {
  server.send(200, "image/png", "");
  WiFiClient client = server.client();
  client.write(image, sizeof(image));
}

void handle_webpage_with_image() {
  server.send(200, "text/html", imagepage);
}
 
void setup(void) {
  Serial.begin(115200);
  Serial.println("");
  pinMode(led, OUTPUT);
  
  // Connect to WiFi network
  WiFi.begin(ssid, password);
  
  // Wait for connection
  while (WiFi.status() != WL_CONNECTED) {
    delay(500);
    Serial.print(".");
  }
  Serial.println("");
  Serial.print("Connected to ");
  Serial.println(ssid);
  Serial.print("IP address: ");
  Serial.println(WiFi.localIP());
   
  // Set up the endpoints for HTTP server
  //
  // Endpoints can be written as inline functions:
  server.on("/", [](){
   server.send(200, "text/html", form);
  });
  
  // And as regular external functions:
  server.on("/adc", handle_adc);
  server.on("/led", handle_led);
  server.on("/led.png", handle_image);
  server.on("/showled", handle_webpage_with_image);
  
  // Start the server 
  server.begin();
  Serial.println("HTTP server started");
}
 
void loop(void) {
  // check for incomming client connections frequently in the main loop:
  server.handleClient();
}

Would appreciate some help getting the handle_image() function to work. (The /adc and / handlers work as intended.)

The esp8266.com link above doesn't help (someone suggests putting the server.send after the client.write, but that doesn't work either.)

In response to /led.png, Firefox gives "The image at blah blah cannot be displayed because it contains errors." Others on esp8266 noted the same error.

If the led.png is saved, it is empty.

void handle_image() {
server.send_P(200, "image/png", image, sizeof(image));
}

You can download a conversion tool from this link:
http://www.codeproject.com/Tips/845393/Convert-a-Binary-File-to-a-Hex-Encoded-Text-File

I can confirm that 'server.send_P()' as suggested by dorin-sima works fine in edge (windows 10), firefox (windows 10, android), chrome (android)

Thanks for this post!

server.send(200, "text/plain", "This is an index page.");
I don't know what is "200" mean
Please help me to understand

200 is a http response code, here's more info:
https://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html

Solo funciona de esta manera:
void handle_image() {
WiFiClient client = server.client();
client.write(image, sizeof(image));
server.send(200, "image/png", "");
}
Este orden esta chekeado al 2018. Muchas Gracias

void handle_image() {
server.send_P(200, "image/png", image, sizeof(image));
}

Great Dorin-Sima !
You fix it !
Thanks so much !

Tested on Geekcreit ESP8266, with board as Wemos D1 R1 (led pin is gpio4)