source_request
Ideally every image should have a valid source where it's taken from the internet. This is important because kusubooru is just an imageboard that "collects" and re-represents other people's work. All the credit goes to the artist so there should be a valid link to where the picture is taken or his/her website.
Of course a lot of pictures are posted in other forums or imageboards or maybe we have them on our hard drives and we have forgotten the website we firstly found them. For those cases, the source_request tag should be added.
You can argue that this is an 'annoying' tag as it might need a lot of research to find the original source and 'wipe' this tag. This is true but let's all try to do as 'clean' work as we can now that this site is still small. Remember that it's much easier erasing a certain tag from all images than adding it to each image.
Warning: Changing the rating of a post from explicit to safe and not reverting it back, will get you banned. Read the how to:rate guide before attempting any change to the ratings.