if you use art that isn't yours in your fanfictions, you must ask and credit the original owner to use it! Saying that the art piece isn't yours or that you found the picture on google isn't good enough. We will decline any fanfiction piece that does not have a proper source and it should be noted that your deviation may be deleted since it is against deviantart terms of service to use art other than your own in your works. In worse case scenarios, you're deviantart account may be deleted. You can read more about copyrights here.
If you have no idea where you found the art at, you can use SauceNAO to find the source.
If that doesn't work, you can also reverse image search on google by going to images, then click on the camera icon that pops up in the search bar. Then you can upload the image or paste the image code (right click, then copy image selection on firefox, right click on the image, then copy image url on chrome) into it.
If you still can't find the original link, then do not use the pic at all! Use one of your own drawings, or find a picture where the original artist allows you to use it so long as you give credit and ask them personally to use it. If they do not give credit, DO NOT USE IT!! If you do not know if they allow you to use it even with giving credit, play it safe and do not use their artwork.
Also, please remember that google, photobucket, zerochan, and other image boards are more likely reposts and not the original source!!! Original sources will most likely be on here, pixiv, tumblr, and some other various blogging websites. Please do not use images from pixiv unless you ask the owner, or it says they allow reposts with credit. Many of the users understand basic english, so please ask them!!
If this is too much trouble for you to go through, then do not use any artworks at all. You do not need to have artworks along with your fanfictions.
Here's some more links about this information:
FAQ #8: What are violations of the deviantART copyright policy?
FAQ #157: Can I use things created by other people in my submissions?
FAQ #193: How do I prove that I had permission to use someone's work?
FAQ #257: What sort of permission do I need to use someone else's work?
Thank you for reading and please pass this on~!