You cannot. This is a text based forum. It's anonymous, it'd be a zoo if they were to allow code or images.
Links are OK, you need to up an image to imgur.com and post a link here.
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You can post html as a regular text, and others can copy-paste it into their browsers.
Pictures are not supported here. I think that's because they usually carry EXIF data, which can be private and traceable. You can upload a picture to imgur or some other site that strips off that data, and paste it here same way as other HTML links.