The rules of colorchallenge clearly state that PHOTO PLAGIARISM IS NOT PERMITTED.

Source: https://pixabay.com/de/photos/raindrop/

Not indicating that the content you copy/paste including images is not your original work could be seen as plagiarism.
Here is a post on why this is bad, and here is how easy it is to find allowed photos.

#photography

Please refrain from using #photography for posting art, posting graphics, posting other people's photos without proper attribution or explicitly stating it's not yours.

Repeated use of the #photography tag when posting other people's photos without attribution or indicating it isn't yours could be considered spam. Spam is discouraged by the community and may result in action from the cheetah bot.

If you are the photographer please reply and let us know!

Thank You!

TIP: You can edit any tag but the first!

More Info: Abuse Guide - 2017.