Jane from JGallegosArt when I was new, your question was one that I also had, not being savvy with anything technical, I had no clue how to "copy and paste"....... so, here you go:
In the "Post a reply" blank square, you type in your sentence such as "My new work!!" Then "enter" to start a new line. Right click on your shop name above in blue, you get a list of options, left click on "open in new tab", then way above you'll see a line showing the Etsy page you're on, and also a line showing your shop page, click on your shop page and select an image by right clicking on it, and choose the "copy shortcut" option, then go back to the Etsy page to your "reply" and right click there choosing the "paste" option. Your image should show up in the box right under your sentence.
Hope it helps! A leader in another Team spelled it out for me.