Sorry, it got a bit convoluted. 
But it’s not that tricky.
If you go to any product page you can quickly long-press the main image to bring up the ‘copy image’ option (if you miss it you can refresh and try again). Copy the image, and paste it into your post and it’ll look something like this:
Which will be a bit on the large side. So you can change the size by amending the ‘375x500’ to something like 200x265 or 150x200 to make it smaller.
You can also change ‘image’ into a description too (it shows up when it’s large, and in search results). So:
Becomes:
