Why to NEVER use images without permission.
I often see people ask about using images from a Google search, or if they can just use an image they like that they saw online. Be warned that using such images can be a very expensive lesson. I always knew that they could not be used, but just today learned that it can be even uglier and costlier to use one then I knew. If you Google something like "copyright-infringement-penalties-are-scary" you will find a blog about how someone mistakenly used an image in a blog, that was actually a copyrighted image. The blog it was on had less then 100 hits. They were being sued for 8k, for the usage of the image without permission. Great article and I recommend it well.
There are a lot of comments and within them I found that all images have copyright protection the moment they are made. It used to be that there had to be a copyright notice. but that no longer holds true. If there is a copyright notice, then an infringement is considered willful and the Max statutory award goes from 30k to 150k.
So, if you want to make sure that you do not face a situation like this you have a few choices.
Take your own pictures, or create your own graphics.
Pay for the right to use a image or graphic.
Use a royalty free image site to find an image, and make sure that you read and follow their directions on how the image can be used. (I would also make extensive notes about what site, and the exact URL where you found the image that was claimed to be royalty free.)
Maybe a bit paranoid, but there have been copyrighted images uploaded to such sites. You might be able to resolve the problem by showing that you were using an image in good faith, but would still have to remove it.
Hope this helps on why you have to be careful on using copyrighted materials.