function ggtup1(){
if(b1>5){
b1=1;
}else{
b1++;
}
if(b1==2){
tup.src="img/a" + b1 + ".jpg"
}else{
tup.src="img/a" + b1 + ".jpg"
}
}
setInterval("ggtup1()",4000);
<img id="tup" src="img/a1.jpg" />
function ggtup1(){
if(b1>5){
b1=1;
}else{
b1++;
}
if(b1==2){
tup.src="img/a" + b1 + ".jpg"
}else{
tup.src="img/a" + b1 + ".jpg"
}
}
setInterval("ggtup1()",4000);
<img id="tup" src="img/a1.jpg" />
b1++ ;
if(b1 > 5){
b1 = 1 ;
}
src = **b1**.jpg