UFOs are renders.

5.6. 3D Renders or Illustrations as Source:
Using other people's 3D renders or illustrations (even with permission and no matter in which form they are offered) creates a false impression of your artwork. So 3D renders or illustrations can not be used as sources, unless they are entirely created by yourself. Only acceptable non-illustration brushes and clip art may be used, but you need to mention them as a source too. Don't use an image that has been altered by the source owner nor an entire work created by someone else.

CMYK46 is correct those UFOs are renderings and can't be used. There are many UFOs that are photos on sites like Flickr (see this forum post on how to search on Flickr: http://www.pxleyes.com/forum/viewtopic.php?f=5&t=5644).

I can give you two links which I know are safe to use, this one on Wikipedia: https://en.wikipedia.org/wiki/File:Moonbeam_UFO.JPG and this one on DeviantArt: https://absurdwordpreferred.deviantart.com/art/Flying-Saucer-160295379 I've checked both and I know they are safe to use, the Wikipedia image is in Public Domain and the one on DeviantArt I verified to be a photo of a scale model. You can find more as I mentioned, if you aren't sure if the ones you find are okay just send me a PM and I'll check them for you.

It's an easy fix but make the correction as soon as you can to avoid a warning from the Contest Moderator.

Thank you for this important piece of info, and the links.