Stop! Don't do it. When you find a useful piece of information on Google you don't cite Google, do you? The same thing goes for images. You need to find the source of the image. Right click on the image and select "View Page Source". That should take you to the place where you can identify the source. Blogs or places where people repost aren't likely to be the original source of the image. You can use reverse image search tools like Tin Eye to help you find the original.
Reminder: Citing an image means you are giving credit to the creator (it's not your work). It doesn't give you copyright permission to reuse the image. That's a separate issue.
