There have been many incidents of image theft from Rendo members here in the past....just do a search here in this forum.
Your small size image is still big enough to "upsize" to print quality with some of the newer software out there.
Cropping and saving your original will help you to prove ownership.....but, that is only after finding out about the theft in the first place....there are many, in fact most, that you will never find.

"Borrowed" artwork is being sold in street markets, on the net, etc....your chances of stumbling across your work would be astronomical...

But, back to the original topic of this thread.....by entering that "contest", you are LEGALLY giving your image away....

In youth, we learn....with age, we understand.