Click inside the "reply" field of a post you want to paste your photo to open it.
In another browser tab/window open your item as if you were a buyer (meaning not from your dashboard).
Right-click the image you want to post and choose copy.
Get back to "reply" field and paste your photo - it should be blue/highlighted with 4 square handles (1 on each corner). The handles help to resize/shrink the photo so it doesn't take to much space.
Get back to the tab/window with your item and now copy the hole url link of that item.
Come back to "reply" field and click the link symbol above the field (5th symbol) to open it and into the url field paste your item's link to insert it. (There are some other options: you can add title to photo, choose if it opens in new or current page). Click OK (black) button to exit and if the photo is still highlighted click on any white space of the reply field.
And then, of course, click reply. Hope it works.
It's easier than it seems :)
this is for example:
