Just about two weeks ago, the Google Search By Image feature stopped working for IE9 users.
Google offered a couple workarounds but those workarounds are no longer needed.
Google's Albert C. said in a Google Web Search Help thread that the issue is now fixed. He wrote:
Just a heads up, we've fixed the issue and you should be able to do these searches from the homepage again.
Honestly, I am surprised this one is fixed that quickly. Typically, issues like this can linger on for months and months without being fixed. I guess this was an easy one or bothered a specific Googler that was able to make a difference.
Forum discussion at Google Web Search Help.