It's not possible to remove this because one of your blog images simply appeared in the Yahoo images results. It probably spiked on that day b/c there were a lot of media articles a day or two earlier about ancient palace ruins being discovered in China.
The search phrase for the image search is "palace ruins found". I think it's likely that it's an image on this post:
http://mytravelwanders.com/2012/03/17/palenque/
I found this by following these steps:
1. Click on the spike in the graph
2. Under the Referrers module, click on "Search Engines" (because I saw 2,025 at the right)
3. Click on Yahoo Image Search (1,967 at right) - notice also "palace ruins found" as top result under Search Engine Terms at the right
4. In Yahoo search bar, enter palace ruins found mytravelwanders
The results show the same images from this post:
http://mytravelwanders.com/2012/03/17/palenque/
There isn't really anything we can do about it, but those are the steps I use to track a spike down.