You have to first upload your image to an image hosting site.
This will then give your image it’s own URL, which can then be posted via HTML/Sandbox instructions, or you can post that URL on this thread and someone will post your image for you.
Firefox now has a screenshot app that let’s you just mark the screen to cover the image and save it to their server...and then post online. Not sure how long it lasts, but it works and I have used it as late as today.;-)