gostaria de colocar uma imagem de fundo nesse codigo tipo um papel de parede .

nao quero pagina branca nem cor. queria por uma imagem de de fundo

#include <SPI.h>
#include <Ethernet.h>
boolean incoming = 0;

int rele1 = 0;
int rele2 = 0;
int rele3 = 0;
int rele4 = 0;

#define pin_rly1 2
#define pin_rly2 3
#define pin_rly3 4
#define pin_rly4 5

byte mac[] = { 0xDE, 0xAD, 0xBE, 0xEF, 0xFE, 0xED };
IPAddress ip(10,0,0,188);
EthernetServer server(8087);
void setup()
{
Ethernet.begin(mac, ip);
server.begin();
Serial.begin(9600);
pinMode(pin_rly1, OUTPUT);
digitalWrite(pin_rly1, LOW);
pinMode(pin_rly2, OUTPUT);
digitalWrite(pin_rly2, LOW);
pinMode(pin_rly3, OUTPUT);
digitalWrite(pin_rly3, LOW);
pinMode(pin_rly4, OUTPUT);
digitalWrite(pin_rly4, LOW);
}

void loop()
{
// listen for incoming clients
EthernetClient client = server.available();
if (client)
{
// an http request ends with a blank line
boolean currentLineIsBlank = true;
String str;
while (client.connected())
{
if (client.available())
{
char c = client.read();
str.concat(c);

if(str.endsWith("/1on")) rele1 =1;
else if(str.endsWith("/1off")) rele1 =0;

if(str.endsWith("/2on")) rele2 =1;
else if(str.endsWith("/2off")) rele2 =0;

if(str.endsWith("/3on")) rele3 =1;
else if(str.endsWith("/3off")) rele3 =0;

if(str.endsWith("/4on")) rele4 =1;
else if(str.endsWith("/4off")) rele4 =0;

if (c == '\n' && currentLineIsBlank)
{
client.println("HTTP/1.1 200 OK");
client.println("Content-Type: text/html"); //inicializa o arquivo html
//client.println("Refresh: 1?); // refresh a pagina a cada 1 seg
client.println();
client.println("<H1><center>Moises WebServer</H1></center><br/>");
//<H1> Tamanho da letra
//<center> Centraliza o texto que estiver na sequência
//<br> Quebr a de linha
client.println("<hr width=50%>"); // TRaço centralizado na pagina
client.println();
client.println("<UL>"); // iniciar lista não numerada
if(rele1 == 1)
{
client.println("<br><LI><font color='blue' size='5'><a href=http://192.168.0.188/1off/><i>Desligar Saida 1</i></a>");
//<LI> Item não numerado
//<a href=endereço pagina/>texto a ser apresentado</a> link para outra pagina
digitalWrite(pin_rly1, HIGH);
}
else if (rele1 ==0)
{
client.println("<br><LI><font color='blue' size='5'><a href=http://192.168.0.188/1on/>LIGAR SAIDA 1</a>");
digitalWrite(pin_rly1, LOW);
}
if(rele2 == 1)
{
client.println("<br><LI><font color='blue' size='5'><a href=http://192.168.0.188/2off/><i>Desligar Saida 2</i></a>");
digitalWrite(pin_rly2, HIGH);
}
else if (rele2 ==0)
{
client.println("<br><LI><font color='blue' size='5'><a href=http://192.168.0.188/2on/>LIGAR SAIDA 2</a>");
digitalWrite(pin_rly2, LOW);
}
if(rele3 == 1)
{
client.println("<br><LI><font color='blue' size='5'><a href=http://192.168.0.188/3off/><i>Desligar Saida 3</i></a>");
digitalWrite(pin_rly3, HIGH);
}
else if (rele3 ==0)
{
client.println("<br><LI><font color='blue' size='5'><a href=http://192.168.0.188/3on/>LIGAR SAIDA 3</a>");
digitalWrite(pin_rly3, LOW);
}
if(rele4 == 1)
{
client.println("<br><LI><font color='blue' size='5'><a href=http://192.168.0.188/4off/><i>Desligar Saida 4</i></a>");
digitalWrite(pin_rly4, HIGH);
}
else if (rele4 ==0)
{
client.println("<br><LI><font color='blue' size='5'><a href=http://192.168.0.188/4on/>LIGAR SAIDA 4</a>");
digitalWrite(pin_rly4, LOW);
}
client.println("</UL>");

break;
}
if (c == '\n')
{
currentLineIsBlank = true;
}
else if (c != '\r')
{
currentLineIsBlank = false;
}
}
}
// give the web browser time to receive the data
delay(1);
// close the connection:
client.stop();
}
}