I don't use iTunes's "fetch artwork", so I'm not 100% sure, but unless you are signed in to your iTunes account, artwork is not automatic. Even then, iTunes doesn't always find it.
You can copy-and-paste artwork from the inernet, either Amazon or Google. I use 300x300 pixel images.
For the future, if you have iTunes questions, you might be better off posting them in the iTunes for Mac forum, this is iTunes for Windows. In this situation, it doesn't make any difference, but next time maybe it will.