It's dead simple Roger, honestly.
When you're typing a message like I am now, just at the bottom of this text box is a paper clip icon with the words "Drag files here to attach, or choose files..."
Simply click on 'Choose files', which opens a window where you look for the file you want to upload, click on the file, click 'Open' and that's it. The forum software will do the rest and Robert is your mother's brother.
For example, just click on the small photo below to view a much larger one. And you can even go further - click on the small one and it opens another window with a larger photo in it. Notice that the cursor has a + sign to it, so click on that and the original size photo will open to show all detail: