Unfortunately, this is nothing new. People have been converting things without user consent for as long as people have been uploading content. There are a few things you yourself can do to help the issue.
If you see that someone has done this, or even suspect that someone has been abusing others content, let that person know.
If your content has been abused like this, there are a few things you can try. One is report the files that they are uploaded on, such as mediafire, box, google drive, ect. A lot of times this can remove those files, and even ban a person from using that site. You can also report it to the website it was uploaded on. Tumblr would be an unlikely place to ban a person for this, but I know that nexus does have a strict policy on this kind of thing. It has been reported before, and has been removed before. The problem is that no one is reporting it, and the only true person who can get those files deleted are the original creators.
Like stated above, it's not a crime to be an asshole, but people still are. The best thing you can do it let creators know, and report it.