In some cases copyright is infringed upon and this is very bad. However, it's usually not really as much about copyright as it is about assumption of ownership, and giving credit where credit is due -- especially on a platform that is based on rewarding content.
Leaving a note that the images are not your own is a weak way of doing this, yes, however most community members source each image to give credit to the original photographer, creator, etc. It shows good faith, and will likely lead to more support!