HTML and Javascript issue: How to loop and change the file names

How can I do a loop and change file names a long with it.   I have the current code that works if i want to loop and print out the img src line 10 types.

But i am unsure of how to loop through, and print out the img src and change the file name as well.

For example first loop would print out

img src="http://test.co/media/img/1.png
next loop will print out
img src="http://test.co/media/img/2.png
next loop will print out
img src="http://test.co/media/img/3.png
and so on un till  img src="http://test.co/media/img/10.png

my guess would be  something like this    img src="http://test.co/media/img/" + HTMLstring[i] + ".png"
but it doesn't seem to work. Or i can be totally off.   Thanks for any help in advance.

var HTMLstring = "";
            for(var i = 0; i < 10; i++){
                HTMLstring += '<img src="http://test.co/media/img/1.png">'
            }