It may be a tiny bit different with your mac, but I'm not sure, and my mac is very old.
I start by finding the photo somewhere, possibly google images or some article.
I'll pick one (in google images) that has pixels listed as 900 or under for the width), otherwise you are apt to find yourself with a giant photo. There are usually plenty of them under 900 pixel width.
I double click on the small image on the google image page.
That brings me, in this case I'm using, to a bigger photo in a news site.
I hold down my control button and click on the photo.
That brings up a list of choices:
I click on OPEN IMAGE IN A NEW WINDOW.
The image shows up.
I copy the image URL, in this case -
and put it in the Reply Box
I either manually add [img] in front of that URL and [/img]immediately after it, or do what Tes yeux noir said,
swipe it with my mouse and click on the image button in the Reply Box.
That's it, for me at least.
Don't know if your mac is the same.
Adds, I don't have a left and right click mouse. If you do, I'd follow what Tes said, and use the left one.