By malicious, do you mean inappropriate, or containing a malicious metadata inside the image? The images are probably stripped and/or compressed, removing any m

By malicious, do you mean inappropriate, or containing a malicious metadata inside the image? The images are probably stripped and/or compressed, removing any malicious metadata. You can use machine learning to classify images of various categories to detect inappropriate images. For example, this is what Yahoo open sourced: https://github.com/yahoo/open_nsfw https://github.com/yahoo/open_nsfw Additionally, user-generated content sites will almost always have a way for users to flag inappropriate content. If something's flagged disproportionately, it will be removed.