Replace a string in C#

Hi All,
I am reading one HTML file in C# .there are several <img> tags inside it.I want to replace the src attribute of each image.
i know this can be done using for or foreach loop.pls provide me the code for that.
suppose there are tags in HTML like <img id="img1" src="images/img1.jpg"></br><img id="img2" src="images/img2.jpg">
then it should replace with
<img id="img1" src="uniqueid1"></br><img id="img2" src="uniqueid2">
thanks in advance. 
 
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