I found a wallpaper on the web that I'd like to use as a background on a website I am building for an academic FBLA project. When I perform a reverse image search on it, I find hundreds of results but not one of them seems to be the original posting. It's most likely that the creator uploaded it on a wallpaper site somewhere (without stating any license information) and it proliferated, or that the "creator" cropped and performed other changes to another image he/she found elsewhere on the web (that may or may not be licensed itself). I can't possibly know for sure. Can I use this image for an academic, non-commercial project without knowing the license or obtaining permission? I read somewhere that you might lose the rights to a piece of intellectual property if you fail to enforce your ownership to a large extent, but I'm not sure. If I can, what phrase should I use to describe my rights to use the image?
By the way, I know that the chances of getting sued or something for using this image are nil. The website won't even be published. However, I do need to provide some sort of license information for the competition.
Thanks in advance.
Edit: also, it's a wood background that isn't highly unique or anything. There are no watermarks or any other text.