Hi!
So, you took a photo using your camera app, or did you take a screenshot? If you took a screenshot, go to your "Photos" and hit the little tab at the top; and then hit "Device Folders." You should find your photo somewhere in there: look at "Screenshots" and "Camera," for example. A little rooting and I'm sure you'll find it. Sometimes my photos go in random folders and though they weren't "downloads" per se, they somehow ended up there haha! Good luck!