I can't understand why you aren't able to upload to the Gallery from your Macbook. The upload image button should take you to a dialog box that allows you to select which drive/folder the image is kept on your machine.
For uploading to threads, I use a host website (Flickr, but other hosts are just as good) and copy the image url, pasting it into the insert image icon dialog box.
It would be helpful if you describe the steps you are taking to try and upload to the Gallery.