Well, that's the thing- unless you ask permission *before* you post it, you've taken it without permission. Which is....the definition of stealing
It seems like no big deal to repost a picture, but it is the property of someone else. I'm sure you (or anyone else) isn't intending to do anything wrong, but copyright laws are something that we should all become familiar with.
There are some pictures which are in the public domain, which means that they are available for others to use as long as credit is given. You'd have to know that the photo falls under that category. Just doing a random search and getting a picture probably isn't going to turn up a public domain picture, but rather a copyrighted one.
Over the years, we've worked hard to protect our user's copyrighted photographs around the internet, and we've also had our share of angry people who had their picture taken without permission and reposted to one of our sites.
Sorry to be the bad news bringer!
