you have to upload them yes but once they are say on TM (which is still the internet by the way!) then it's just a case of adding correct tags and url to stick them in a none gallery post topic.
You can't directly link to pictures on your computer (unless yours is a server connected to net) using the file manager - Using TMs image manager in the Gallery allows you to Upload images to TM but others can not post their images in a gallery topic
