To answer your question, it is a real picture. No the person who linked it here did not take it herself, unless her name is Andy Buckley. Stolen? No, on the Internet you can't "steal" anything, but you can make copies, or link to other people's stuff, which has happened here. Fortunately Andy has released the said picture licensed as Creative Commons Zero which basically states the photo:
- is free to copy and use for personal and commercial purposes
- doesn't require attribution
So basically no laws were breached here. I'm not sure if #colorchallenge is for personal photographs or not, but I'd guess it's personal. In that case it would be a bit questionable, if not outright dumb to post other people's photographs to the challenge.