If the site shows a profile picture for you that you did not upload, it is due to you being a guest. We utilize technology that assigns a mostly unique device identifier to every guest to allow them to experience features such as uploading a photo.
Since your device and another guest user may share the exact same screen size, operating system, browser, plugins, your identifier may match theirs. So their guest photo will show up for you.
To change the photo, see our photo upload help article.