You can obviously only affect what is in the browser, not the source file.
Here are several examples that demonstrate what I think you wnat to do:
#include <ie.au3>
; 1
$oIE = _IE_Example()
sleep(2000)
_IEBodyWriteHTML($oIE, '<IMG src="http://www.autoitscript.com/images/download_autoit.png">')
; 2
$oIE = _IE_Example()
sleep(2000)
$oImg = _IETagnameGetCollection($oIE, "img", 0)
_IEPropertySet($oImg, "outerhtml", '<IMG src="http://www.autoitscript.com/images/download_autoit.png">')
; 3
$oIE = _IE_Example()
sleep(2000)
$oImg = _IETagnameGetCollection($oIE, "img", 0)
$oImg.src = "http://www.autoitscript.com/images/download_autoit.png"
Dale