You don't upload an image to the site but you hyperlink in an image.
Find a image you want to share on the web. Right click on it and choose an option like "Copy image address". When you want to post it here click on the IMG tab in the vB Code bar and paste in your image address.
I suggest using the Preview Reply button to make sure it works, and the "reload" button in your browser to make sure the site that is hosting the image allows remote linking. Then choose "submit reply".