If you upload an image here at the English Wikipedia, then it will be available for use on this site. However there are many other language versions of Wikipedia, as well as a variety of sister projects, which may also want to use your image. Therefore, it is generally better to upload your free images to '''[[commons:Main Page|Wikimedia Commons]]''', a shared site where an image can be uploaded once and then used by any of these projects.
There is an important caveat: '''Commons only accepts free images. It does not accept non-free or fair-use images.''' Images must also be free in both the country of origin and the United States.
To upload images to Commons you can use the '''[[commons:Special:UploadWizard|Commons Upload Wizard]]''' tool, which will guide you through the process.
