First of all, you must have a program or app that can take a screenshot of the image on your screen. A good recommendation would be the Snipping Tool. Next, after editing the image to make sure it meets the policies of the wiki, you can begin the uploading process. In most wikis, there is an "Photos" option at the top of the screen. From that point, there should be a place where you can upload the image you have created. Follow the steps listed, and you should be able to upload images onto a wiki.