I have been using images from places like WIkimedia where the licence allows re-use, but I also link back to where they came from. I think that's okay. You can use the Tools > Usage Rights options in Google to find such images. I try to use my own images when possible, but it does help to have some image on a post to get people to notice it in the feeds.
I've not had anyone say I'm doing it wrong so far.