The topic does have an interesting balance in terms of copyright protection in terms of intellectual property; in most cases it must be patented/trademarked depending on the exact nature of it. 

End of the day the reuse of images is so rife in our society that it's pointless to argue that writers should never use images made by someone else if they don't explicitly say not to. Look at the comparison for memes, how many of us share and repost memes, yet never reference their original creator?

The underlying issue I see is there is often a misunderstanding from the artists perspective of when these images are used, they're somehow being claimed by the person using them rather them then going oh look, here's an image that perfectly suits the world/character I'm trying to create. That is based off my experience interacting with artists who have joined to complain about the misuse. They can't be surprised at the image being reused as it is in the public domain and often has no watermark or private use statement. I've used other images from google searches like this in the past because they capture that setting/flavour of the idea I may feel I'm not properly portraying in words.

However end of the day, this isn't a in court legal argument, it's more a commonality of respect among artists; we don't claim credit for other people's work and if the original artist has an issue with it, then we remove it.

On the topic of people being reprimanded over the use of plagiarism in contests, that is because there was prewarnings on the contest entries that it had to solely be your own work for the single purpose of the contest. Additionally this was done for the purposes of gain by cheating the system.