User blog comment:Tanglefrost/dumb question i probably shouldn't have wasted a blog on, but oh well/@comment-9949103-20150426123351

Go to pixlr.com, launch the pixlr-o-matic app, upload the picture you want to greyscale, select the filter called "Tom", and then save the picture to your computer. Once that is done, you should be able to simply upload the greyscaled picture to the wiki :)