The following is an example. After measurement, you can optimize it as needed:

& lt！ DOCTYPE html & gt

& lthtml & gt

& lthead & gt

< title & gt menu

& ltscript type = " text/JavaScript " & gt；

Function chgimg(src)

{

var img = document . getelementbyid(" img ")；

img.src=src+"。 jpg "；

}

& lt/script & gt；

& lt/head & gt；

& ltbody & gt

& ltdiv & gt

& ltp & gt& ltimg src = " a 1 . jpg " id = " img "/& gt； & lt/p & gt；

& ltp & gt& ltul & gt& ltLi onclick = " chg img(' a2 ')；" & gta</ Li> & ltlionclick = "chgimg ('a3');" & gtb</ Li>& lt Li onclick = "chgimg ('a1');" & gtc</ Li>& lt/ul & gt；; & lt/p & gt；

& lt/div & gt；

& lt/body & gt；

& lt/html & gt；