Work is underway on SN to permit images posts. But for now, you can create a Description post, and then use markdown.

And you can do a "larger image" link like this:

SN won't host the image, so you need something like Imgur or PostImages.org. The link needs to be the image's url itself, not the link for the page on those platforms. i.e., upload or whatever, then when you have page with your image, right click on that image and copy image address -- that gives you the link to use with markdown.

This markdown for images works in comment replies as well. And the image can be a .gif animation too.