For social networking sites (Google+, Facebook, etc. ), you can find your pictures saved inside the photo album section of your account. Open your account profile and you should see your photo albums inside. For forum sites, the photo you want to be deleted can be found on the discussion thread you shared it on before. Navigate through the forum site and look for the thread where you posted the picture.
For some sites, like online forums, you may have to edit your post (using the edit tool of the site, if any is present) and manually delete it from the site to remove the picture from the Internet.
