asp:image problem

 Hi. I have an <asp:Image src="source1" /> in my form.
I change the "src" of this image on client using javascript to "src=source2" then i post the form back to server.
My problem is that the "src" of my image is still "source1" and not "source2" when i post the form to the server, pressing the "Done" button.

I know i can do postback and change the "src" on the server and this way it is preserved but i don't whant to do that because postbacks takes a lot of time to complete.
 Any workaround?

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